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 it is 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.