The word _relation_ is commonly used in two senses considerably different from each other. Either for that quality, by which two ideas are connected together in the imagination, and the one naturally introduces the other, after the manner above explained; or for that particular circumstance, in which, even upon the arbitrary union of two ideas in the fancy, we may think proper to compare them. In common language, the former is always the sense in which we use the word relation; and 'tis only in philosophy that we extend it to mean any particular subject of comparison, without a connecting principle. Thus, distance will be allowed by philosophers to be a true relation, because we acquire an idea of it by the comparing of objects: but in a common way we say, _that nothing can be more distant than such or such things from each other, nothing can have less relation_; as if distance and relation were incompatible. It may, perhaps, be esteemed an endless task to enumerate all those qualities, which make objects admit of comparison, and by which the ideas of _philosophical_ relation are produced. But if we diligently consider them we shall find, that without difficulty they may be comprised under seven general heads, which may be considered as the sources of all philosophical relation. 1. The first is _resemblance_: and this is a relation, without which no philosophical relation can exist, since no objects will admit of comparison, but what have some degree of resemblance. But though resemblance be necessary to all philosophical relation, it does not follow that it always produces a connexion or association of ideas. When a quality becomes very general, and is common to a great many individuals, it leads not the mind directly to any one of them; but, by presenting at once too great a choice, does thereby prevent the imagination from fixing on any single object. 2. _Identity_ may be esteemed a second species of relation. This relation I here consider as applied in its strictest sense to constant and unchangeable objects; without examining the nature and foundation of personal identity, which shall find its place afterwards. Of all relations the most universal is that of identity, being common to every being, whose existence has any duration. 3. After identity the most universal and comprehensive relations are those of _space_ and _time_, which are the sources of an infinite number of comparisons, such as _distant, contiguous, above, below, before, after, &c_. 4. All those objects, which admit of _quantity_ or _number_, may be compared in that particular, which is another very fertile source of relation.