David Hume is famous for making us realize that until we know the Necessary Connection between cause and effect all human knowledge is uncertain, merely a habit of thinking based upon repeated observation (induction), and which depends upon the future being like the past. On the basis of the casual principle we bind together the passing impressions into universal laws and if this principle is valid, then there in rational justification for science, otherwise, Hume points out, we have no reason to accept science’s claims as knowledge.
It is said that a cause produces an effect, or, the effect is necessarily connected with a cause. In order to show that that the idea of cause be true and real, it must be, according to Hume, traced to some impressions. But which impressions are these? Hume believes that to answer this question two more questions must first be asked:
- For what reason we pronounce it necessary, that everything whose existence has a beginning, should also have a cause?
- Why do we conclude, that such particulars causes must necessarily have such particular effects, and what is the nature of that inference we draw from the one to the other?
Hume claims that neither intuitively certain nor demonstrable is the philosophical maxim that whatever begins to exist must have a cause. He does not regard the necessity of necessary condition, as all distinct ideas are separable from each other. The idea of cause and effect are distinct from each as one can conceive of an object as non-existent at one moment and existent in the next without conjoining to it the idea of a cause or a productive principle. Hume then attacks the proofs of causality given by Locke, Hobbes and Clarke. Hume rejects Locke’s argument that a thing which come into being without a cause is caused by nothing, for nothing cannot be the cause of something, or anything for that matter. According to him, every demonstration which has been produced for the necessity of cause is fallacious and sophistical; instead of proving causality, really presupposes it.
Answering the second question, Hume explains how causes and effects are discovered, not by reason but through experience, when we find that particular objects are constantly conjoined with one another. The idea of cause must be derived from the relation among objects. We tend to overlook this because most ordinary causal judgments are so familiar; we’ve made them so many times that our judgment seems immediate. But when we consider the matter, we realize that “an (absolutely) inexperienced reasoner could be no reasoner at all”. Even in applied mathematics, where we use abstract reasoning and geometrical methods to apply principles we regard as laws to particular cases in order to derive further principles as consequences of these laws, the discovery of the original law itself was due to experience and observation, not to a priori reasoning. Even after we have experience of causal connections, our conclusions from those experiences aren’t based on any reasoning or on any other process of the understanding. They are based on our past experiences of similar cases, without which we could draw no conclusions at all. These experiences which one may draw from a cause to an effect or from an effect to a cause, have to be based on one’s experience of a constant conjunction, or of a regular succession of cause and effect, or of the contiguity in time or place of the cause and effect.
But this leaves us without any link between the past and the future. How can we justify extending our conclusions from past observation and experience to the future? The connection between a proposition that summarizes past experience and one that predicts what will occur at some future time is surely not an intuitive connection; it needs to be established by reasoning or argument. The reasoning involved must either be demonstrative, concerning relations of ideas, or probable/moral, concerning matters of fact and existence.
There is no room for demonstrative reasoning here. We can always conceive of a change in the course of nature. However unlikely it may seem, such a supposition is intelligible and can be distinctly conceived. It therefore implies no contradiction, since there is no contradiction in suggesting that the future will not resemble the past. Moral/Probable reasoning can’t establish the connection, either, since it is based on the relation of cause and effect. What we understand of that relation is based on experience and any inference from experience is based on the supposition that nature is uniform — that the future will be like the past. Appeal to experience will then either be circular or question-begging. For any such appeal must be founded on some version of the uniformity principle itself — the very principle we need to justify.
For induction to be a valid form of reasoning is must be founded in the ‘uniformity principle’. If something hasn’t changed till today, why should it change tomorrow? When we examine experience to see how expectations are actually produced, we discover that they arise after we have experienced “the constant conjunction of two objects;” only then do we “expect the one from the appearance of the other.” But when “repetition of any particular act or operation produces a propensity to renew the same act or operation…we always say, that this propensity is the effect of Custom”. Our belief in induction is not based in reason but in custom.
Past experiences have led us to believe in certain things about future events. Belief requires that there also be some fact present to the senses or memory, which gives “strength and solidity to the related idea.” In these circumstances, belief is as unavoidable as is the feeling of a passion; it is “a species of natural instinct,” “the necessary result of placing the mind” in this situation. Belief is “a peculiar sentiment, or lively conception produced by habit” that results from the manner in which ideas are conceived, and “in their feeling to the mind.” Reason is far weaker tool than we might have supposed. Belief is “nothing but a more vivid, lively, forcible, firm, steady conception of an object, than what the imagination alone is ever able to attain”. Belief is thus “more an act of the sensitive, than of the cogitative part of our natures”, so that “all probable reasoning is nothing but a species of sensation”.
“There are no ideas, which occur in metaphysics, more obscure and uncertain, than those of power, force, energy or necessary connexion”. Hume wants to “fix, if possible, the precise meaning of these terms, and thereby remove some part of that obscurity, which is so much complained of in this species of philosophy”. Does something known as necessary connection, which philosophers so talk about, exist? When we examine a single case of two events we regard as causally related, our impressions are only of their conjunction; the single case, taken by itself, yields no notion of their necessary connection. When we go beyond the single case to examine the background of experienced constant conjunctions of similar pairs of events, we find little to add, for “there is nothing in a number of instances, different from every single instance, which is supposed to be exactly similar”. How can the mere repetition of conjunctions produce a necessary connection?
While there is indeed nothing added to our external senses by this exercise, something does happen: “after a repetition of similar instances, the mind is carried by habit, upon the appearance of one event, to expect its usual attendant, and to believe that it will exist.” We feel this transition as an impression of reflection, or internal sensation, and it is this feeling of determination that is “the sentiment or impression from which we form the idea of power or necessary connexion. Nothing farther is in the case.
Hume sums up all of the relevant impressions in not one but two definitions of cause. The first definition, which defines a cause as “an object, followed by another, and where all objects similar to the first are followed by objects similar to the second”, accounts for all the external impressions involved in the case. His second definition, which defines a cause as “an object followed by another, and whose appearance always conveys the thought to that other” captures the internal sensation — the feeling of determination — involved. Both are definitions, by Hume’s account, but the “just definition” of cause he claims to provide is expressed only by the conjunction of the two: only together do the definitions capture all the relevant impressions involved.
Hume’s account of causation provides a paradigm of how philosophy, as he conceives it, should be done. He goes on to apply his method to other thorny traditional problems of philosophy and theology: liberty and necessity, miracles, design. In each case, the moral is that a priori reasoning and argument gets us nowhere: “it is only experience which teaches us the nature and bounds of cause and effect, and enables us to infer the existence of one object from that of another. Such is the foundation of moral reasoning, which forms the greater part of human knowledge, and is the source of all human action and behaviour”