Saturday, April 13, 2013

David Hume: easy overview

David Hume – An Enquiry Concerning Human Understanding
Section I
Hume opens the Enquiry by drawing a distinction between two kinds of philosophical thinking. The first he calls an "easy and obvious philosophy" which serves as a guide for the man of action. Usually written in an elegant and poetic style, this philosophy tries to cultivate our manners by drawing on examples from common life and making us feel the difference between vice and virtue. This philosophy excites the sentiments and leads us to assent to a way of life that we know to be good. Such philosophy--which Hume associates with figures such as Cicero, La Bruyere, and Addison--is generally quite popular and keeps good standing in posterity. It follows from common sense and thus rarely falls into error.
This philosophy is contrasted with the "accurate and abstract philosophy" of the man of reason. Rather than direct our behavior, this philosophy seeks to form our understanding and to uncover the principles that govern our behavior. Rather than rely upon common sense, this philosophy--which Hume associates with figures such as Aristotle, Malebranche, and Locke--proceeds by means of abstract reasoning from the particular to the general. This kind of philosophy has little application once one leaves off philosophical contemplation. Because its reasoning and conclusions often go against common sense, error in this field is not at all uncommon, and as a result it fares rather poorly in posterity and is sometimes rejected altogether.
Common wisdom suggests that this accurate and abstract philosophy is not to be disregarded entirely, but that a good life consists in an appropriate mix of different elements. The philosopher is often looked down upon for being too shut off from the world outside, but those who ignore philosophy entirely are even more despised for their ignorance. We are reasonable beings, and thus aspire to scientific knowledge, but this knowledge is limited. We are also social and active beings, though a purely social life can become tiresome and a life devoted purely to business and action can wear us out. The "easy and obvious" philosophy, then, is often considered an ideal that appropriately combines philosophical reflection with a more active and social life. A life dedicated only to the pursuit of scientific knowledge is usually punished with pensive melancholy, uncertainty, and public disapprobation.
Nonetheless, Hume argues that a careful study of this accurate and abstract philosophy has its virtues. It calls for an exactness and accuracy that can lead to perfection in more practical matters. For instance, the scientific study of anatomy may seem grotesque in itself, but a painter can create beautiful and anatomically precise figures through careful application of its principles. Besides, Hume remarks, on its own, such scientific study is harmless, is good exercise for the mind, and can help bring us to the truth.
The best objection Hume admits against accurate and abstract philosophy is that it is not science, but rather a confused attempt to explain by means of blind prejudice what we do not know. However, Hume notes, this is not a reason to abandon philosophy, but an exhortation to study it more carefully. If we can properly explain the nature and principles that govern human understanding, as Newton has done for the principles that govern planetary orbits, we can reject bad reasoning and proceed more carefully. Though the mental faculties are most present to us, they are very difficult to conceive of precisely and we are still far from uncovering the fundamental principles that we seek. Still, this is only further reason to study them, and while we can often make mistakes through faulty reasoning, this fallibility is hardly just cause to abandon the project altogether.
This first section lays out the framework of Hume's project. He is clearly greatly influenced by the scientific method and empirical philosophy. His stated goal is to do for the mind what Newton has done for matter. Before Newton, we could explain a great deal about planetary orbits and even predict how and where things would move. However, until Newton, we were unable to explain why the planets move as they do. Newton's theory of gravitation gives a clear and simple explanation as to why the planets orbit the sun and why objects on the earth fall toward its center. Hume believes our explanations of human understanding and behavior are in a state similar to that of pre- Newtonian astronomy. We can observe a great deal about how we think and can often reason quite fruitfully, but as yet we have no clear grasp of the principles that underlie our thought and reasoning.
Hume's stated method is scientific, of careful observation and inference from particular instances to general principles. The drive of scientific inquiry is to dig deeper and deeper so as to uncover a very few, very simple principles that govern all the complexities that we observe. Newton's genius gives us three very simple laws that can explain and predict all physical phenomena. Hume wishes to perform a similar feat for human understanding (the word "understanding" is used by Hume to describe most broadly the several faculties of human reason). The hope is that Hume will derive a similarly small and simple number of principles that can explain and predict the processes of human thought. His method will be to proceed from simple observation of how the mind works and how we use it in everyday life, and to infer from his observations increasingly general principles that govern our understanding until he reaches a bedrock of simplicity and clarity.
In this respect, Hume follows very much in the empiricist vein of philosophy and owes a large debt to ##John Locke##. Locke moved against rationalist philosophy, best exemplified by ##Descartes##, which relies heavily upon rational intuition. The empiricist tradition asserts that experience, and not reason, should serve as the basis of philosophical reasoning.
The motivation for Hume's project is made apparent in his complaint that the "accurate and abstract" metaphysics that he is pursuing is frequently looked down upon and disdained. The difficulty and counter-intuitive nature of these inquiries often lead to errors that may seem absurd and prejudicial to future generations. Even today, there is a great deal of debate as to whether there has been any real "progress" in philosophy: we may have refined our discussions and dismissed some bad ideas, but in essence we are still mulling over the same problems that concerned Plato and Aristotle. It would not be unreasonable to suggest that we are no nearer a satisfactory and final answer than the ancient Greeks. Hume hopes that scientific observation can uncover the principles that underlie our reasoning so that we can be more immediately aware of faulty logic and more easily guided along the correct path.
Ever since the scientific revolution of Newton, Galileo, and others, science has been held up as a paradigm of fruitful reasoning. In science, there is a carefully defined methodology that precisely details how we can test a theory and determine whether it is right or wrong. Though it is often difficult to determine the right answer, the scientific method usually prevents us from arriving at answers that are far from the mark. Philosophy lacks any such determinate method, and philosophers are continually taking up conflicting views. For instance, Hume's emphasis on observation goes directly against Descartes' rationalism, which disparages observation in favor of pure reason. Hume hopes that his empiricism will open the way for a carefully defined method that will not allow for such disparity amongst philosophers.
Hume also suggests that his work must be epistemically (epistemic: of, relating to, or involving knowledge; cognitive) prior to the new science that he so lauds. The scientific method is a product of careful reasoning, and is thus subject to the laws of human understanding. While science seems to be in far better shape than philosophy, it too can benefit from his work. In this way, Hume differs from his predecessor, Locke. Locke sees himself as laboring on behalf of the new science, clearing away some of the linguistic rubble that might lead to confusion. While Locke humbly sees himself as simply clearing a path for science, Hume believes that his own work must lay the groundwork upon which science can rest. If he can uncover the precise laws that govern our reasoning and inferences, this should help us draw the right conclusions in our scientific investigations.

Sections II and III
Hume draws a distinction between impressions and thoughts or ideas (for the sake of consistency, we will refer only to "ideas" from here on). Impressions are lively and vivid perceptions, while ideas are drawn from memory or the imagination and are thus less lively and vivid. Impressions comprehend, according to Hume, "all our more lively perceptions, when we hear, or see, or feel, or love, or hate, or desire, or will." Thus, both the color red and the feeling of anger are considered impressions. Ideas are what arise when we reflect upon our impressions, so the memory of seeing the color red or a thought about anger are considered ideas.
While we might consider the human mind an unlimited organ, able to conceive of imaginary creatures and far-off lands with great facility, Hume points out that our imagination in fact consists merely of a complex of ideas. For instance, if we imagine a gold mountain, we are compounding our idea of gold with our idea of a mountain. Hume provides two arguments to support this claim. First, he suggests that all complex ideas are compounded out of simple ideas, which are in turn derived from simple impressions. For instance, our idea of God as supremely good and intelligent comes from taking our simple ideas of human goodness and intelligence and augmenting them without limit. Second, he points out that our imagination is limited to those ideas of which we have had impressions. Thus, a blind man is unable to imagine colors, a deaf man to imagine sounds, or a mild-mannered man to imagine cruelty.
Hume admits one objection to his distinction. He points out that I can imagine certain colors without ever having perceived them. For instance, if I have seen several shades of blue, I might be able to imagine some other shade of blue that falls between them. Though he has no answer to this objection, he remarks that the counter-example is so singular that is does not upset his general maxim.
This distinction between impressions and ideas is valuable for clearing up our philosophical vocabulary. While ideas are faint, obscure, and easily confounded with other ideas, impressions are vivid and clearly defined, and we are not likely to fall into error with respect to them. Thus, when we find ourselves discussing a philosophical term that we suspect may not refer to any idea, we may simply ask from what impression its supposed idea might be derived. Since all ideas are derived from impressions, a term that is not connected to any impression is meaningless.
In a footnote, Hume notes that his distinction between impressions and ideas clears up some confusions found in Locke's rejection of innate ideas. Hume complains that Locke fails to clarify what he means either by "innate" or "idea." In Hume's vocabulary, we could assert that impressions are innate and ideas are not.
In section III, Hume discusses the connections that exist between ideas, asserting that all ideas are linked to other ideas. Hume lays out three principles by which ideas might be associated: resemblance (where a picture of a tree might make us think of the tree), contiguity in time or place (where mention of one apartment might lead us to discuss others), and cause and effect (where the thought of a wound makes us think of the pain that follows from it). Hume admits that he has no reason for laying out only these three principles except that he cannot think of any others that would be needed. For instance, association by means of contrast or contrariety can be seen as a combination of resemblance and causation.
Here we begin to see Hume's empirical method at work. By looking inward and observing his own mental processes, Hume brings to bear three important distinctions. The first, and most important, is the distinction between ideas and impressions. This distinction is original to Hume and solves a number of difficulties encountered by Locke. A proper discussion of Hume's footnote would take us too far afield, but we should remark that Hume's criticism of Locke is exact and powerful. The distinction between impressions and ideas might seem quite obvious and of no great importance, but Hume is quite clever to identify the full importance of this distinction. An empirical philosophy asserts that all knowledge comes from experience. For Hume, this would suggest that all knowledge comes from impressions, and so ideas are set up as secondary to impressions.
The second distinction, between complex and simple impressions or ideas, helps draw out further the power of the first distinction. A simple impression might be seeing the color red, while a complex impression might be seeing the totality of what I see right now. A simple idea might be the memory of being angry while a complex idea might be the idea of a unicorn (composed of the idea of a horse and the idea of a horn). Complex ideas and impressions are compounded out of the simple ones.
With these first two distinctions, Hume is creating a hierarchy of mental phenomena. Since the complex is compounded out of the simple and ideas are derived from impressions, everything in our mind is based ultimately upon simple impressions. A complex idea is compounded out of several simple ideas, which are in turn derived from several corresponding simple impressions. Hume thus suggests that a term can only be meaningful if it can be connected with an idea that we can associate with some simple impressions. Hume, we should note, is silently implying that every term must be connected with some idea. In the eighteenth century the philosophy of language had not yet flourished, and it was not clear how difficult it might be to determine precisely how words, ideas, and reality link up. Hume's suggestion that all terms can be analyzed into simple impressions anticipates Russell, who argues that we can analyze all terms into simple demonstratives like "this" or "that." Hume's suggestion comprehends a picture of language according to which the words we use are a complex and opaque expression of a simpler underlying language which proper analysis can bring out.
The third distinction is the three laws of association. If the previous two distinctions give us a geography of the mind, describing its different faculties, this distinction gives us a dynamics of the mind, explaining its movement. According to Hume, any given thought is somehow related to adjacent thoughts just as any given movement in the physical world is somehow related to adjacent moving bodies. His three laws of association, then, might be seen as equivalent to Newton's three laws of motion. With them, Hume hopes to have described fully the dynamics of the mind.
There are a number of objections we might want to raise to Hume's distinctions and the way they are introduced, but we will touch on only a few briefly. First, we might ask how strictly we can distinguish between impressions. Hume argues that ideas can be vague, but that impressions are exact and that the boundaries between them are clearly defined. Is the boundary between the impression of a 57" stick and a 58" stick that clearly defined? There is some level of vagueness in our impressions that Hume does not acknowledge. We could also point out that while we are experienced in distinguishing colors, we are not so good with some other sensations. For instance, we often have trouble distinguishing between tastes.
Second, we might object to Hume's implicit philosophy of language. It seems closely linked to the idea that simple impressions are clearly defined and infallible. It is far from clear, however, why it should be desirable or possible to reduce all our language to simple impressions. What, we might ask, is the simple impression from which is derived the word "sake," for example?
Third, we might ask Hume to be clearer in his distinctions. For instance, are dream images impressions or ideas? Most likely they are ideas, since they consist of a mixture of imagination and memory. However, dreams are (arguably) phenomenally indistinguishable from waking experience: we cannot prove that we are dreaming from within a dream. Thus, all our impressions from within a dream are as real to us as we dream them as waking impressions are to us when we experience them.

Section IV
Hume opens this section by drawing a distinction between "relations of ideas" and "matters of fact." Relations of ideas are a priori and indestructible bonds created between ideas. All logically true statements such as "5 + 7 = 12" and "all bachelors are unmarried" are relations of ideas. Relations of ideas are intuitively or demonstrably certain, and a denial of such a proposition implies a contradiction.
Matters of fact deal with experience: that the sun is shining, that yesterday I went for a walk, or that it will rain tomorrow are all matters of fact. They are learned a posteriori, and can be denied without fear of contradiction. If it is sunny outside and I assert that it is raining, I can only be proven wrong by looking out the window and checking: my assertion cannot be disproved simply by an appeal to logic and reason.
While I may know many matters of fact from sensory experience or from memory, neither is the source of my knowledge that my friend is in France or that the sun will rise tomorrow. Hume suggests that we know matters of fact about unobserved things through a process of cause and effect. My knowledge that my friend is in France might have been caused by a letter to that effect, and my knowledge that the sun will rise tomorrow is inferred from past experience, which tells me that the sun has risen every day in the past.
Hume then asks how we know the principle of cause and effect: if I see one billiard ball rolling toward another, how do I know that the second ball will move when it is struck? He suggests that this knowledge cannot be a priori, since I can deny that the second billiard ball will move without contradiction. Cause and effect are themselves totally distinct: nothing in the movement of the first billiard ball can a priori suggest to me the movement of the second billiard ball. Hume thus concludes that our knowledge of cause and effect must be based on experience. From observed phenomena in the past we infer as yet unobserved phenomena in the future.
We base our knowledge of future events in past experience, but how do we know that the past is a good guide for future predictions? Hume distinguishes between "demonstrative reasoning," which is based on relations of ideas, and "moral reasoning," which is based on matters of fact. We cannot know that the future will resemble the past by means of demonstrative reasoning, since there is no contradiction in suggesting that the future will not resemble the past. Moral reasoning is also unhelpful, since it falls into a vicious circle. If all our predictions about the future are based on this principle--that the future will resemble the past--and that principle is derived from past experience, we cannot know that it will remain true in the future except by assuming that principle from the outset.
Hume suggests that we infer similarities between past and future but that there is no form of reasoning that can confirm these inferences. He confesses that he may simply have failed to identify an argument that could give a rational foundation for causal reasoning, but he challenges the reader to identify it. Even a child knows from past experience that a flame will burn. If this knowledge comes from some form of reasoning, it must be a form of reasoning so obvious that even a child can grasp it. Why, then, Hume asks, is it so difficult to identify? He suggests that the child learns, not through reasoning, but through the conditioning of custom.
Hume's distinction between relations of ideas and matters of fact is one of the first formulations of a distinction that has been instrumental in philosophy ever since. Kant made the distinction famous, calling relations of ideas "analytic" and matters of fact "synthetic." Ever since, and particularly in the development of analytic philosophy in the twentieth century, the analytic/synthetic distinction has been a hot topic of debate.
It might not be clear what Hume means when he says it would be a contradiction to deny relations of ideas, but not matters of fact. Surely, there is something contradictory about saying "it's raining" when the sun is shining brightly. The point is that we need to refer to the world around us to verify matters of fact. The claim that two plus two equals five is a contradiction because nothing in our experience can possibly prove it true. The claim "it's raining" might have been true under other circumstances, and the claim must be compared with reality in order to be proven false.
We can know relations of ideas quite easily by means of what Hume calls demonstrative reasoning. There are well-established axioms and rules of inference according to which I can derive mathematical and other logical truths. Similarly, there are well-established means of knowing observable matters of fact. For instance, my claim that it's raining can be verified by stepping outside or looking out the window. However, Hume notes that unobserved matters of fact are more difficult to sort out. I know that the sun will rise tomorrow, but how? I won't be able to verify that claim directly until tomorrow, but I can still claim to know it with certainty today.
Hume suggests that we use the principle of cause and effect to reason through matters of fact. The principle of cause and effect, he suggests, we learn from experience. The question, then, is how we can ground general principles that we learn from experience. This question cuts right to the very heart of the inductive reasoning that is central both to the scientific method and to Hume's philosophy. All our general principles in philosophy and science are induced from particular examples. Induction essentially consists in observing and predicting the future based on what we have observed in the past. We are absolutely certain that the second billiard ball will move when it is struck, not through demonstrative reasoning, but because we have seen bodies collide in that way countless times during our lives and have never seen one instance to the contrary.
For induction to be a valid form of reasoning, we need to propose some sort of "uniformity principle" that establishes that the future will resemble the past. It may seem obvious that physical laws will not change in the future, but Hume's genius lies precisely in seeing that this is still a claim that needs to be proved and argued for. To his surprise, he finds there is no good reason to trust in any sort of uniformity principle. It cannot be established through reason alone, since its denial is hardly contradictory. It would seem that we learn of this principle through experience, but we cannot claim that it is confirmed in experience. A uniformity principle is needed to justify all inductive claims based on past experience, so we cannot prove the uniformity principle itself through induction. We need to prove the uniformity principle before we can say anything about induction or knowledge from experience, but it seems that we cannot prove the uniformity principle without an appeal to experience. This circularity could be schematized as follows:

Our knowledge from experience is based on the principle of cause and effect

The principle of cause and effect is grounded in induction

Induction relies on the uniformity principle, that the future will resemble the past

We come to know the uniformity principle from experience

If we ask how we ground our knowledge from experience (and hence the uniformity principle) we return to (1) and our reasoning has come full circle.
Rather than try to hedge at this point, Hume bites the bullet and accepts the consequences of his reasoning: there is no way we can prove any kind of uniformity principle, and so induction is not a valid form of reasoning. Any reasoning about future events is mere conjecture and the claim that the sun will rise tomorrow is no more certain than the claim that aliens will invade the earth tomorrow. Hume is not necessarily claiming that there is no uniformity principle or that there is a good chance that the sun will not rise tomorrow. He is saying that if there is some hidden power that enforces a continued regularity in physical laws, it is beyond the power of our reason to detect it. Our belief in induction is not based in reason but simply in custom. Past experience has led us to believe certain things about future events (and indeed, this experience rarely leads us astray) but these beliefs are not rationally justified. Hume's argument is that we are committed to the belief that the future will resemble the past, but that we are not rationally justified in holding this belief. Reason is a far weaker tool than we might have supposed.

Section V
Hume acknowledges that the skepticism employed in the previous section could never undermine our reasoning from common life: nature always wins out against abstract reasoning. However, he does claim to have shown that there is a step in our reasoning from experience that is not supported by any argument or process of understanding. There is no solid reason why we should reason according to cause and effect, and yet we never fail to do so.
Hume notes that someone thrown into the world with no prior experience would have no understanding of the process of cause and effect. Life would be an unintelligible string of unconnected events. We cannot sense causation, nor (as Hume has argued in the previous section) is it present to our reason. Hume's answer is that our inductive reasoning regarding experience is derived from custom and not from the understanding. This is why we need to see a process recur many times before we can begin to see two events in the process as causally connected. I need only examine the diagram of one circle in order to derive through reason the properties that all circles share in common. However, we must see many collisions of billiard balls and other objects before custom can implant in us the inference that the movement of one object is causally connected to the movement of another.
Without custom, Hume remarks, reasoning that concerns matters of fact could not extend beyond memory and present sense experience. We could not speculate nor even act if custom had not implanted in us the ability to see certain actions as having certain consequences. Nonetheless, Hume points out, all reasoning from experience ultimately falls back upon simple impressions. What I know about past ages might come from reading a history book, or what I speculate about the future might ultimately fall back upon observations I am making in the present. Our speculations about unobserved matters of fact rest upon a constant conjunction with our present impressions.
Hume suggests that we make inferences by means of the imagination, but draws a careful distinction between fiction and belief. Fiction is the product of pure imagination by means of which we can conjure up all sorts of strange images derived from our simple impressions, such as unicorns, alien civilizations, and what have you. Belief is a combination of imagination and a certain sentiment that we cannot control that suggests to us that our imaginings correspond with reality. When some memory or sense impression is present to our minds, the force of custom will then carry the imagination to think of something to which that impression is constantly conjoined. This force of custom forms our beliefs, and creates a more vivid, forceful, and firm version of our pure imaginings.
Cause and effect, like the other two laws of association discussed in section III, allow the mind to move from one thought to another. When these laws of association are led by custom, they form very strong instinctive beliefs. Hume remarks that it is fitting that our knowledge of causation should be formed by instinct rather than by reason. It is very important that we see the world causally, since it is the source of all action and speculation, and reason is too unreliable a tool. Young children have still unformed reasoning abilities, and even intelligent adults commit countless errors in their reasoning. The instincts enforced by custom are far less susceptible to error, and are thus a far stronger means of securing our knowledge of cause and effect.
The terms "skepticism" and "naturalism" are frequently mentioned in discussions of Hume, and his relationship with each is hotly debated. Hume is termed a skeptic on account of the doubts he raises as to the capabilities of reason. The classic account of modern skepticism is found in Descartes' ##Meditations##, in which all knowledge based on sensory experience is cast into doubt. We might read Hume as going even farther, casting our ability to reason inductively into doubt. While Descartes ultimately squirms away from his doubts, Hume sticks to his, claiming that we have no rational justification for anything outside of immediate sensations and a priori reasoning.
Essentially, Hume doubts the rational foundation of everything that is useful and helps us get by in the world. All action and speculation is based upon suppositions of cause and effect. If I did not think my actions would have any consequences, I would not act. For instance, I go to work because I know I will get money if I do so, and I help my friend because I know my friend will be better off if I do so. If I had no reason to expect any consequences from my actions, I would have no reason to go to work, to help my friend, or anything else besides.
Hume's line of skepticism cuts to the very heart of our conception of ourselves as rational beings. He makes us question what we mean when we say we do things for a reason. In fact, his argument seems to imply, what we call our reasons are not reasons at all, or at least they are not rationally justified.
This brings us to the naturalist line in Hume's philosophy. While Hume denies that we have reasons for believing or acting as we do, he also explains the causes for our behavior and our actions. He argues that induction and causal reasoning are implanted in us by custom and constant conjunction. In replacing reason with custom, Hume reconceives the nature of human thought and action. Most philosophy, in particular the rationalist philosophy of Descartes, sees human beings as primarily rational animals, informed and guided by reason. Hume's reconception sees us more as creatures of custom and habit, much like the animals we so frequently try to set ourselves above.
While Hume's discussion of custom and constant conjunction may seem odd to us, it is in fact just another way of framing something that should be relatively clear. Both Hume and the traditional philosopher would agree that certain events invariably follow certain other events, and both Hume and the traditional philosopher would agree that our behavior is largely dictated by our knowledge of this sequence. The difference lies in the fact that the traditional philosopher would then argue that there is some principle of cause and effect that we know and can see in operation between two connected events. Hume denies that we know any such principle, suggesting instead that habit simply implants an expectation in us that events will fall out in a certain pattern. He uses the term "constant conjunction" to suggest that we cannot say that two events are causally related, but only that we constantly find one followed by the other.
The harsh limitations that Hume sets upon reason might lead us to question the validity of science and the scientific method that Hume holds so dear. An interesting and significant point is that only philosophy, and not science, ever makes any claim regarding the certainty of causal reasoning. All scientific knowledge comes from experience, but science is also careful never to assert the certainty of this knowledge. For instance, Newton's three laws all come from induction: he observes that certain events invariably follow upon one another, and draws up laws to explain this constant conjunction. However, all laws of physics are nothing more than hypotheses. A physical theory, we could argue, can never be proved, but only disproved. Every piece of evidence in its favor only serves to make it more likely, but no evidence could ever make it certain.
Only philosophy, in its yearning for certainty, has tried to suggest that there is such a thing as a law of cause and effect. Science rests content in making predictions based on experience without claiming any kind of certainty or privileged reasoning to back these predictions up. Hume might then also defend his own philosophy, saying that he proceeds according to a similar method.

Section VI and Section VII, Part 1
Section VI is a short section entitled "Of Probability." Hume asserts that there is no such thing as chance in the workings of the universe, but that our ignorance of the real causes of events leads us to a belief in chance. Hume conjectures that belief differs from fiction simply in this: what we believe is more forcefully imprinted upon our imagination because it is more likely to arise. Belief, Hume asserts, is just what is confirmed by experiment.
In section VII, "Of the Idea of Necessary Connection," Hume suggests that no idea in metaphysics is more obscure and uncertain than what is variously termed "force," "power," "energy," or "necessary connection." As he has argued in section II, all ideas and complex impressions are initially formed by simple impressions, which are vivid, sensible, and unambiguous. For a complex idea like causation to have any meaning, we must be able to trace it from the simple impression from whence it is derived.
Hume argues that there is no simple impression that could inform us of necessary connection. He examines in turn our impressions of interactions between two bodies, between mind and body, and within the mind, and argues that in each case we do not perceive, by experiment or reason, any secret power of necessary connection.
Hume has already discussed the body-body interaction of billiard balls. All we observe is that the motion of the first billiard ball is followed by the motion of the second billiard ball: we cannot observe the act of causation. Nor does the mind perceive the workings of cause and effect: otherwise we could determine what effects would follow from causes without ever having to rely on observation.
Next, Hume looks at the mind-body interactions according to which an act of volition can cause the movement of limbs. Hume points out that while we are aware of our ability to move our body, we are by no means aware of the connection between the act of volition and the bodily movement. The connection between mind and body is poorly understood at best, nor do we understand why we are so capable of moving, say, our fingers but not of controlling, say, our heart. Furthermore, Hume points out that there is a long chain of muscle and nerve reactions between the act of volition and the movement of the body. Our mind wills that the arm should move, but it actually produces a whole series of effects which it in no way wills.
Finally, Hume looks at mind-mind interactions, whereby we focus the mind or produce ideas, and fails to locate any necessary connection. First, he points out that we are unaware of how the mind can conjure an idea out of nothing. Further, he points out that experience teaches us that the mind has varying degrees of control, so that it has more power over reason than the passions, or that it has greater self-command when it is healthy. That we learn these things from experience suggests that we are observing only a constant conjunction and not some necessary connection.
Hume goes on to examine and attack the occasionalist picture, which suggests that what we perceive as "causes" are in fact "occasions" and that God is the ultimate cause of all change. Considering the limitations of the human intellect, Hume ponders what stroke of logic could possibly produce such unsupported and outlandish conclusions. Further, he questions how we might know the forces that are operated by the mind of God if we cannot even decipher the forces that are operated by our own minds and bodies.
The motivation behind Hume's brief discussion of probability might not be readily apparent. Considering his emphasis on the scientific method, we should not be surprised that Hume carries a deterministic worldview: nothing that happens happens purely by chance. We may not be able to predict the outcomes of dice rolls, but this is simply because we cannot adequately calculate all the relevant factors. Hume will address the difficult question of how free will might be reconciled with this determinism in section VIII.
Hume's determinism should suggest to us that his skepticism is epistemological and not metaphysical. That is, Hume does not believe that it is pure coincidence that billiard ball collisions always happen in the same way. Rather, he believes that we are incapable of rationalizing the causal connection. We might read Hume as saying that everything that happens happens according to some sort of law or necessity, but that these laws or necessities are beyond our understanding.
We invent the notion of probability and chance, Hume suggests, because we cannot actually determine precisely how things will happen. These probabilities are determined by experience. For instance, if car crashes kill passengers 80 percent of the time, I will judge it highly probable that a car crash will result in death. Other probabilities are 100 percent certain: for instance, flames always burn. This certainty does not then result from observing directly some power of causation or necessary connection, but comes instead from a calculation of probability based on experience. Hume's discussion of probability explains his tendency to see reasoning about matters of fact as determined by habit and experience rather than by an understanding of causation.
A little more straightforward in its intent and methodology, section VII returns to the central line of Hume's argument in the Enquiry. This section is meant to establish what precisely we mean when we talk about causation.
Before we continue, perhaps we should clear up the distinction between causation and necessary connection. Generally speaking, we can say that A causes B if B temporally succeeds A, if A and B are spatially contiguous, and if B always follows A. However, if I always hum while striking a match, we can hardly say that my humming causes the match to burst into flame, even though it satisfies all the above criteria. Causation must also rely on some kind of necessary connection: the match could still burst into flame if I didn't hum, but it couldn't burst into flame if I didn't strike it. The question for Hume, then, is how we can know or perceive this necessary connection. What is it about the striking of the match and not my humming that connects to the match's bursting into flame?
The first part of section VII could be read as the negative phase of Hume's argument. With body-body, mind-body, and mind-mind interactions, Hume shows that there is no evidence of necessary connection. If we knew of necessary connection purely through reason, we would not need experience to show us that two events are necessarily connected. However, in each case, Hume shows us that it is experience that teaches us of this connection. Furthermore, we do not actually experience the necessary connection itself: we only infer it from the constant conjunction that we observe between two events. Here, Hume's discussion of probability comes in once more. We observe that in 100 percent of cases, one billiard ball striking a second billiard ball is followed by the movement of the second billiard ball. This observation leads us to infer that there must be some necessary connection between the collision and the movement of the second ball even though we cannot directly observe that connection.
That all ideas and complex impressions are derived from simple impressions is central to Hume's thinking. For our idea of necessary connection to have any coherence, it must be related to some simple impression. However, Hume's arguments show us that there is no simple impression that produces the idea of necessary connection. As we mentioned before, Hume is not suggesting that it is pure coincidence that the second billiard ball invariably moves when it is struck. Instead, he is suggesting that whatever causal connection there might be between the two events cannot be rationalized by us.
At the end of the first part of section VII, Hume touches on occasionalism, most famously represented by Nicolas Malebranche (1638-1715). A proper discussion of occasionalism is beyond the scope of the present commentary, but we should note that Hume displays a great deal of intellectual courage and integrity in not shying away from the skeptical consequences of his argument. Unfortunately, he was rewarded for this with accusations of atheism that plagued him his entire life.

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