In Section 4 of An Enquiry Concerning Human Understanding (click the link to get it free for the kindle), David Hume relentlessly and overwhelmingly attacks human knowledge. His skeptical questions concerning how we know what we (think we) know is so thorough and so devastating that it’s almost depressing to read it. First, Hume goes after causation. Here are some instructive quotations:
“All reasonings concerning matter of fact seem to be founded on the relation of Cause and Effect. By means of that relation alone we can go beyond the evidence of our memory and senses…A man finding a watch or any other machine in a desert island, would conclude that there had once been men in that island. All our reasonings concerning fact are of the same nature. And here it is constantly supposed that there is a connexion between the present fact and that which is inferred from it. Were there nothing to bind them together, the inference would be entirely precarious. The hearing of an articulate voice and rational discourse in the dark assures us of the presence of some person: Why? because these are the effects of the human make and fabric, and closely connected with it. If we anatomize all the other reasonings of this nature, we shall find that they are founded on the relation of cause and effect, and that this relation is either near or remote, direct or collateral. Heat and light are collateral effects of fire, and the one effect may justly be inferred from the other. If we would satisfy ourselves, therefore, concerning the nature of that evidence, which assures us of matters of fact, we must enquire how we arrive at the knowledge of cause and effect. I shall venture to affirm, as a general proposition, which admits of no exception, that the knowledge of this relation is not, in any instance, attained by reasonings a priori; but arises entirely from experience, when we find that any particular objects are constantly conjoined with each other…”
To summarize: all reasoning about the real world is founded on the notion of cause and effect, and we know about this only from our experience. Continuing:
“It must certainly be allowed, that nature has kept us at a great distance from all her secrets, and has afforded us only the knowledge of a few superficial qualities of objects; while she conceals from us those powers and principles on which the influence of those objects entirely depends…
“It is allowed on all hands that there is no known connexion between the sensible qualities and the secret powers; and consequently, that the mind is not led to form such a conclusion concerning their constant and regular conjunction, by anything which it knows of their nature. As to past Experience, it can be allowed to give direct and certain information of those precise objects only, and that precise period of time, which fell under its cognizance: but why this experience should be extended to future times, and to other objects, which for aught we know, may be only in appearance similar; this is the main question on which I would insist. The bread, which I formerly eat, nourished me; that is, a body of such sensible qualities was, at that time, endued with such secret powers: but does it follow, that other bread must also nourish me at another time, and that like sensible qualities must always be attended with like secret powers? The consequence seems nowise necessary.”
I think Hume is correct to say that bread providing nourishment is not necessarily true. However, if one has eaten bread on a very large number of occasions and always observed it to provide nourishment, then this means that most or all bread provides nourishment (if bread provided nourishment half the time, or one time in ten, then it would be very unlikely that you would have observed it providing you nourishment every single time in the past, assuming you’ve consumed bread a few hundred times or so), which means that you are justified in believing the next piece of bread you eat will provide nourishment (it probably will, since it is an individual from a population that is mostly or totally made up of individuals that provide nourishment). At the time Hume did not know how it was that bread provides nourishment. Of course, nowadays we have nutritional science and can give a good explanation. That being said, there are still plenty of cases where we can’t give an explanation of why it is that one event, which we call an effect, follows from a previous event, which we call the cause. Our ignorance about this much is true. Though this ignorance cannot be totally dissolved, it is my suspicion that in at least a few cases the effect necessarily results from the cause. Let’s take one of Hume’s examples: that of a Billiard ball communicating motion to another billiard ball after it is hit. Given the law of conservation of energy, energy cannot disappear, and if it cannot disappear then the only thing for it to do is to continue into the next billiard ball (there are few exceptions to the communication of energy rule, and even those have explanations consistent with conservation of energy). I suspect the law of conservation itself is necessary: if, from moment to moment, zero energy is added to or subtracted from existence (as must be the case, since no energy can exist outside of existence) then it follows that the amount of energy in the universe must remain constant.
After questioning again how one might justify inductive/causal reasoning, Hume says that he doesn’t think there will be a genuine justification to the problem:
“With regard to the present subject, there are some considerations which seem to remove all this accusation of arrogance or suspicion of mistake. It is certain that the most ignorant and stupid peasants—nay infants, nay even brute beasts—improve by experience, and learn the qualities of natural objects, by observing the effects which result from them. When a child has felt the sensation of pain from touching the flame of a candle, he will be careful not to put his hand near any candle; but will expect a similar effect from a cause which is similar in its sensible qualities and appearance. If you assert, therefore, that the understanding of the child is led into this conclusion by any process of argument or ratiocination, I may justly require you to produce that argument; nor have you any pretence to refuse so equitable a demand. You cannot say that the argument is abstruse, and may possibly escape your enquiry; since you confess that it is obvious to the capacity of a mere infant. If you hesitate, therefore, a moment, or if, after reflection, you produce any intricate or profound argument, you, in a manner, give up the question, and confess that it is not reasoning which engages us to suppose the past resembling the future, and to expect similar effects from causes which are, to appearance, similar. This is the proposition which I intended to enforce in the present section. If I be right, I pretend not to have made any mighty discovery. And if I be wrong, I must acknowledge myself to be indeed a very backward scholar; since I cannot now discover an argument which, it seems, was perfectly familiar to me long before I was out of my cradle.”
I think Hume has, once again, gone wrong. The fact that inductive and causal reasoning are like instincts (or “customs” as Hume put it) does not mean that there can be no justification for them, even if “brute beasts” and babies employ such instincts without knowing the justification. An analogy is in order: for quite a long time, people have been using intuitive methods of reasoning in science, history, and ordinary life which seem to approximate Bayes’ Theorem. For so long, no one knew the logical justification for such reasoning, and it is only now, over 200 years after its discovery, that people are now beginning to realize this. People’s ignorance of such a justification did not mean that there was none. As an interesting sidenote, it does suggest that natural selection has molded our intuitions to conform to principles justified by Bayes’ Theorem. I’ll end there, and we’ll pick back up in the next installment.