Since it appears, that the transition from an impression present to the memory or senses to the idea of an object, which we call cause or effect, is founded on past _experience_, and on our remembrance of their _constant conjunction_, the next question is, whether experience produces the idea by means of the understanding or imagination; whether we are determined by reason to make the transition, or by a certain association and relation of perceptions. If reason determined us, it would proceed upon that principle, _that instances, of which we have had no experience, must resemble those of which we have had experience, and that the course of nature continues always uniformly the same._ In order, therefore, to clear up this matter, let us consider all the arguments upon which such a proposition may be supposed to be founded; and as these must be derived either from _knowledge_ or _probability_, let us cast our eye on each of these degrees of evidence, and see whether they afford any just conclusion of this nature. Our foregoing method of reasoning will easily convince us, that there can be no _demonstrative_ arguments to prove, _that those instances of which we have had no experience resemble those of which we have had experience_. We can at least conceive a change in the course of nature; which sufficiently proves that such a change is not absolutely impossible. To form a clear idea of any thing is an undeniable argument for its possibility, and is alone a refutation of any pretended demonstration against it. Probability, as it discovers not the relations of ideas, considered as such, but only those of objects, must in some respects be founded on the impressions of our memory and senses, and in some respects on our ideas. Were there no mixture of any impression in our probable reasonings, the conclusion would be entirely chimerical: and were there no mixture of ideas, the action of the mind, in observing the relation, would, properly speaking, be sensation, not reasoning. 'Tis therefore necessary, that in all probable reasonings there be something present to the mind, either seen or remembered; and that from this we infer something connected with it, which is not seen nor remembered.