因果の必然性 : ヒュームの因果論

Abstract

We usually refer to causal necessity in two different manners. First, we say that effect is necessitated in a physical way, i. e., effect is forcibly produced by virtue of "efficacy" or "power" inherent in cause. Secondly, we say that effect is necessitated in a logical way, i. e., effect is logically derivable from cause. The first kind of necessity is physical necessity, and the second kind a logical one. David Hume made a great contribution to the study of the problem of causation. Hume's arguments as to the nature of causation are divided into two parts, negative or critical and positive. It is well known how in his negative or critical arguments Hume rejected both kinds of necessity. He shows that the connection of cause and effect is not logically necessary. He also shows that we can never perceive any causal efficacy. But, at the same time, in his positive arguments he says that causal necessity lies in the psychological inevitability which our minds experience when we make causal reasonings. According to Hume, when we have experienced a constant conjunction of two sorts of objects, we call the one sort of objects causes and the other effects. And when one object occurs new which belongs to the one sort of the constantly conjoined objects, we can never fail to expect the occurrence of the other object which belongs to the other sort of them. In Humean phrases, one object "determines the mind to conceive the idea of other object." This "determination of the mind, " he says, is the product of the association between the ideas of the two objects which stand in the relation of constant conjunction, and moreover, surprisingly, the source of the idea of causal necessity, "the idea of necessary connection of cause and effect", which is rejected in his critical arguments. In other words, causal reasonings consist of our continuing to believe that the constant conjunction which we have experienced in the past will be found also in the future, i. e., our continuing to rely on the principle of uniformity of nature. If this principle is presupposed, causal reasonings become logically valid, but since that principle itself cannot be demonstrated, our reasonings never ensure the truth of the conclusion, and thus causal reasonings, which are of the same nature as inductive arguments, are not justified. In short, Hume's final conclusion is that constant conjunction constitutes the essence of causation. This is known today as "the regularity or uniformity view of causation." In view of some intractable difficulties arising from his way of treating with the matter, I should like suggest the following interpretation of Hume's view of causal necessity. First, we formulate a proposition stating a particular causal law, which is analysable into the experienced conjunction of two particular objects and the principle of uniformity of nature. Secondly we formulate a factual proposition(A) reporting the occurrence of one of the objects and a factual proposition (B) reporting the occurrence of the other Then, the factual proposition (B) is logically nessary to the proposition stating the particular causal law and the factual proposition(A). Thus, from the proposition stating that causal law and the factual proposition(A), we are entitled to derive the factual proposition(B). Causal Necessity, then, means this very derivability which is the logical relation of entailment between propositions.