I believe it will not be very necessary to prove, that these qualities produce an association among ideas, and, upon the appearance of one idea, naturally introduce another. 'Tis plain, that, in the course of our thinking, and in the constant revolution of our ideas, our imagination runs easily from one idea to any other that _resembles_ it, and that this quality alone is to the fancy a sufficient bond and association. 'Tis likewise evident, that as the senses, in changing their objects, are necessitated to change them regularly, and take them as they lie _contiguous_ to each other, the imagination must, by long custom, acquire the same method of thinking, and run along the parts of space and time in conceiving its objects. As to the connexion that is made by the relation of _cause and effect_, we shall have occasion afterwards to examine it to the bottom, and therefore shall not at present insist upon it. 'Tis sufficient to observe, that there is no relation, which produces a stronger connexion in the fancy, and makes one idea more readily recal another, than the relation of cause and effect betwixt their objects. That we may understand the full extent of these relations, we must consider, that two objects are connected together in the imagination, not only when the one is immediately resembling, contiguous to, or the cause of the other, but also when there is interposed betwixt them a third object, which bears to both of them any of these relations. This may be carried on to a great length; though, at the same time we may observe, that each remove considerably weakens the relation. Cousins in the fourth degree are connected by _causation_, if I may be allowed to use that term; but not so closely as brothers, much less as child and parent. In general, we may observe, that all the relations of blood depend upon cause and effect, and are esteemed near or remote, according to the number of connecting causes interposed betwixt the persons. Of the three relations above mentioned this of causation is the most extensive. Two objects may be considered as placed in this relation, as well when one is the cause of any of the actions or motions of the other, as when the former is the cause of the existence of the latter. For as that action or motion is nothing but the object itself, considered in a certain light, and as the object continues the same in all its different situations, 'tis easy to imagine how such an influence of objects upon one another may connect them in the imagination.