In his Essays on the Active Powers of Man, Reid calls into question Hume’s treatment of the concept of cause. Reid points to Hume’s theory as being one that denies that we have a conception of an efficient cause. After providing his own account of Hume’s definition of causal necessity as constant conjunction, Reid criticizes Hume’s theory on the grounds that . Reid writes:
But theory ought to stoop to fact, and not fact to theory. Every man who understands the language knows, that neither priority, nor constant conjunction, nor both taken together, imply efficiency…The very dispute, whether we have the conception of an efficient cause, shows that we have. For though men may dispute about things which have no existence, they cannot dispute about things of which they have no conception. (IV.2, p. 279)
According to Reid, the notion of efficiency is closely tied to “the exertion of an active power” (IV.2, p. 276). This active power serves as a cause to produce an effect. Reid tells us that we are, from an early age, aware of the fact that we possess this active power and that it is through our possessing this power that we are the causes of our voluntary actions. I find it interesting that Reid takes his theory, unlike Hume’s, to be one that “stoops to fact”. Indeed, there appears to be something to Reid’s criticism, that is, it does seem to be the case that the notion of constant conjunction does not logically imply efficiency. However, it seems that Reid may be missing an important aspect of Hume’s theory of necessity and his approach to how one is to provide definitions to the terms one uses to theorize.
I take it that in response to Reid’s criticism Hume could counter by claiming that it is Reid that does not remain faithful to the facts. For Hume, the existence of a necessary causal relation amongst objects does not arise merely from two objects being constantly conjoined in a certain temporal order, it also importantly involves the inference of the mind, i.e. the mind’s passing from one object to the other, from which we “infer the existence of one from that of the other”. (Treatise 2.3.1, Par. 4) Hume’s response to Reid might involve his pointing to the fact that, indeed, we do appear to have a conception of an “efficient cause”. And, it seems that we use this notion in our theorizing. However, upon closer examination, we are to see that if we carefully attend to our use of the terms ‘necessity’, ‘efficiency’, or ‘causality’, the meaning of these terms ultimately reduces to the circumstances upons which we employ them, i.e our observance of the constant conjunction of objects and the inference of the mind.
Thus, contrary to Reid’s suggestion, Hume does not think that we fail to have actual “conceptions” of these terms; Hume merely thinks that their meaning must have some content that is readily accessible to us in experience. And so, while Reid takes his theory to remain faithful to the “facts”, it remains to be seen what Reid takes these facts to be. If the facts are simply that we use notions like ‘necessity’, ‘efficiency’ and ‘causality’ in our theorizing, then he is in agreement with Hume. However, Hume would likely respond to this criticism by claiming that it is Reid’s theory that does not “stoop to the facts” given that Hume’s theory actually seems better supported by the facts at hand.