A Treatise of Human Nature by David Hume — Read

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 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.