In A Treatise of Human Nature, Hume draws strong similarities between the causal interactions of material bodies and the connections between human actions and their motives. Hume emphasizes that reason “detects” the necessary causal connections between objects in both cases in the same way. Moreover, Hume tells us that the necessity of the connection between a purported cause and effect consists merely in their “constant union” and in “the inference of the mind” which arises from our observation of this union (2.3.1, Par. 4). After Hume’s discussion of how this works in the case of external bodies, he tells us that he also wishes to show that this story also holds between our actions and our motives. Hume tells us that:
…in judging of the actions of men we must proceed upon the same maxims, as when we reason concerning external objects. When any phaenomena are constantly and invariably conjoin’d together, they acquire such a connexion in the imagination, that it passes from one to the other, without any doubt or hesitation. (2.3.1, Par. 12)
While Hume is keen to draw this similarity between our reasoning about nature and our reasoning about human action, I find it difficult to see how Hume can get to the strong conclusions he wishes to derive concerning moral responsibility given what he admits in the case of our knowledge of material bodies. Hume seems to admit that our understanding of them is importantly limited. Hume writes that:
…[i]t has been observ’d already, that in no single instance the ultimate connexion of any objects is discoverable, either by our sense or reason, and that we can never penetrate so far into the essence and construction of bodies, as to perceive the principle, on which their mutual influence depends. ‘Tis their constant union alone, with which we are acquainted; and ‘tis from the constant union the necessity arises. (2.3.1, Par. 4)
Here, it seems that Hume is admitting that while we have the idea of a necessary connection between material causes and effects, we will never truly know the nature of the “ultimate connexion” between them. It would seem, then, that Hume ought to apply this epistemic modesty to claims concerning our knowledge of the “ultimate connexion” between human actions and motivations. Given that reason detects connections amongst these types of “objects” in the same way, our knowledge in both cases should, presumably, be of the same kind.
In the Enquiry Conerning Human Understanding, however, Hume seems to treat our knowledge of such a connection with respect to actions and motives as privileged in some respect. Hume tells us that one may try to counter his particular treatment of the necessary connection between actions and motives by challenging his treatment of that between material bodies. One could maintain, Hume suggests, that it’s “possible to discover something farther in the operations of matter.” However, Hume provides a response to this possible strategy by arguing that the truth of this claim:
…can be of no consequence to morality or religion, whatever it may be to natural philosophy or metaphysics. We may here be mistaken in asserting, that there is no idea of any other necessity or connexion in the actions of body: but surely we ascribe nothing to the actions of the mind, but what every one does, and must readily allow of. We change no circumstance in the received orthodox system with regard to the will, but only in that with regard to material objects and causes. (8.2, Par. 27)
I guess I have two issues with how Hume seems to counter this possible response to his theory of the causal relations amongst actions and motives. The first is with his seeming to draw a distinction between “natural philosophy or metaphysics” and morality, and the second is his failure to acknowledge that one could also try to counter his treatment of necessity by supposing that it is possible to discover something further about the workings of the operations of the human mind, or will.
In the first case it seems that Hume relies fairly heavily on the type of “necessary connection” between material bodies in order to make his point concerning the connection between action and motivation. It seems odd that Hume would, at this point, pull apart one’s theorizing about nature from one’s theorizing about human nature and action. Moreover, in the second case, it seems that Hume does not take seriously the possibility that we might discover something further about the operations of the human mind, something that may undermine the justifiability of our inference from a person’s actions to their motivations or character. Given Hume’s epistemic modesty with respect to the essence of material bodies and the real connections between them, it seems that Hume ought to exercise the same type of modesty in the case of the relation between actions and motives, and recognize the possibility of such a discovery and its posing a threat to his theory of moral responsibility.