It has been observed already, that in no single instance the ultimate connexion of any objects is discoverable either by our senses 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. If objects had not an uniform and regular conjunction with each other, we should never arrive at any idea of cause and effect; and even after all, the necessity which enters into that idea, is nothing but a determination of the mind to pass from one object to its usual attendant, and infer the existence of one from that of the other. Here then 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. As the actions of matter have no necessity but what is derived from these circumstances, and it is not by any insight into the essence of bodies we discover their connexion, the absence of this insight, while the union and inference remain, will never, in any case, remove the necessity. 'Tis the observation of the union which produces the inference; for which reason it might be thought sufficient, if we prove a constant union in the actions of the mind, in order to establish the inference along with the necessity of these actions. But that I may bestow a greater force on my reasoning, I shall examine these particulars apart, and shall first prove from experience that our actions have a constant union with our motives, tempers, and circumstances, before I consider the inferences we draw from it. To this end a very slight and general view of the common course of human affairs will be sufficient. There is no light in which we can take them that does not confirm this principle. Whether we consider mankind according to the difference of sexes, ages, governments, conditions, or methods of education; the same uniformity and regular operation of natural principles are discernible. Like causes still produce like effects; in the same manner as in the mutual action of the elements and powers of nature. There are different trees which regularly produce fruit, whose relish is different from each other; and this regularity will be admitted as an instance of necessity and causes in external bodies. But are the products of Guienne and of Champagne more regularly different than the sentiments, actions, and passions of the two sexes, of which the one are distinguished by their force and maturity, the other by their delicacy and softness?