I now proceed to explain the _second_ part of my system, and show why the constancy of our perceptions makes us ascribe to them a perfect numerical identity, though there be very long intervals betwixt their appearance, and they have only one of the essential qualities of identity, viz. _invariableness_. That I may avoid all ambiguity and confusion on this head, I shall observe, that I here account for the opinions and belief of the vulgar with regard to the existence of body; and therefore must entirely conform myself to their manner of thinking and of expressing themselves. Now, we have already observed, that however philosophers may distinguish betwixt the objects and perceptions of the senses; which they suppose co-existent and resembling; yet this is a distinction which is not comprehended by the generality of mankind, who, as they perceive only one being, can never assent to the opinion of a double existence and representation. Those very sensations which enter by the eye or ear are with them the true objects, nor can they readily conceive that this pen or paper, which is immediately perceived, represents another which is different from, but resembling it. In order, therefore, to accommodate myself to their notions, I shall at first suppose that there is only a single existence, which I shall call indifferently _object_ or _perception_, according as it shall seem best to suit my purpose, understanding by both of them what any common man means by a hat, or shoe, or stone, or any other impression conveyed to him by his senses. I shall be sure to give warning when I return to a more philosophical way of speaking and thinking. To enter therefore upon the question concerning the source of the error and deception with regard to identity, when we attribute it to our resembling perceptions, notwithstanding their interruption, I must here recal an observation which I have already proved and explained.[5] Nothing is more apt to make us mistake one idea for another, than any relation betwixt them, which associates them together in the imagination, and makes it pass with facility from one to the other. Of all relations, that of resemblance is in this respect the most efficacious; and that because it not only causes an association of ideas, but also of dispositions, and makes us conceive the one idea by an act or operation of the mind, similar to that by which we conceive the other. This circumstance I have observed to be of great moment; and we may establish it for a general rule, that whatever ideas place the mind in the same disposition or in similar ones, are very apt to be confounded. The mind readily passes from one to the other, and perceives not the change without a strict attention, of which, generally speaking, 'tis wholly incapable.