All those objects, of which we call the one _cause_ and the other _effect_, considered in themselves, are as distinct and separate from each other as any two things in nature; nor can we ever, by the most accurate survey of them, infer the existence of the one from that of the other. 'Tis only from experience and the observation of their constant union, that we are able to form this inference; and even after all, the inference is nothing but the effects of custom on the imagination. We must not here be content with saying, that the idea of cause and effect arises from objects constantly united; but must affirm, that 'tis the very same with the idea of these objects, and that the _necessary connexion_ is not discovered by a conclusion of the understanding, but is merely a perception of the mind. Wherever, therefore, we observe the same union, and wherever the union operates in the same manner upon the belief and opinion, we have the idea of causes and necessity, though perhaps we may avoid those expressions. Motion in one body, in all past instances that have fallen under our observation, is followed upon impulse by motion in another. 'Tis impossible for the mind to penetrate farther. From this constant union it _forms_ the idea of cause and effect, and by its influence _feels_ the necessity. As there is the same constancy, and the same influence, in what we call moral evidence, I ask no more. What remains can only be a dispute of words. And indeed, when we consider how aptly _natural_ and _moral_ evidence cement together, and form only one chain of argument betwixt them, we shall make no scruple to allow, that they are of the same nature, and derived from the same principles. A prisoner, who has neither money nor interest, discover the impossibility of his escape, as well from the obstinacy of the gaoler, as from the walls and bars with which he is surrounded; and in all attempts for his freedom, chooses rather to work upon the stone and iron of the one, than upon the inflexible nature of the other. The same prisoner, when conducted to the scaffold, foresees his death as certainly from the constancy and fidelity of his guards, as from the operation of the axe or wheel. His mind runs along a certain train of ideas: the refusal of the soldiers to consent to his escape; the action of the executioner; the separation of the head and body, bleeding, convulsive motions, and death. Here is a connected chain of natural causes and voluntary actions; but the mind feels no difference betwixt them in passing from one link to another; nor is less certain of the future event than if it were connected with the present impressions of the memory and senses by a train of causes cemented together by what we are pleased to call a _physical necessity_. The same experienced union has the same effect on the mind, whether the united objects be motives, volitions and actions, or figure and motion. We may change the names of things, but their nature and