In both the Treatise and the Enquiry Hume claims that the most original or interesting part of his contribution to free will rests with his definition or understanding of what we mean by necessity (T 2.3.1.18, 2.3.2.4/407, 409–10; see also EU 8.1–3, 8.21–25/80–81, 92–96). It is this issue, Hume claims, that has been the primary obstacle to resolving this controversy. According to Hume there are “two particulars, which we are to consider as essential to necessity, viz. the constant union and the inference of the mind; and wherever we discover these we must acknowledge a necessity” (T 2.3.1.4/400). In order to explain this, Hume begins with a description of causation and necessity as we observe it in “the operations of external bodies” (T 2.3.1.3/399) or in “the actions of matter” (T Abs. 34/ 661). Here we find “not the least traces of indifference or liberty” and we can see that “[e]very object is determin’d by an absolute fate” (T 2.3.1.3/400). What this means, Hume explains, is that we discover that there exist constant conjunctions of objects, whereby resembling objects of one kind are uniformly followed by resembling objects of another kind (e.g., Xs are uniformly followed by Ys). (See, in particular, T 1.3; T Abs. 8–9, 24–26/649–50, 655–57; and also EU 4 and 7). When we experience regularities of this sort we are able to draw relevant inferences, and we deem objects of the first kind causes and those of the second kind their effects.