These are therefore the principles of union or cohesion among our simple ideas, and in the imagination supply the place of that inseparable connexion, by which they are united in our memory.  Here is a kind of attraction, which in the mental world will be found to have as extraordinary effects as in the natural, and to shew itself in as many and as various forms.  Its effects are every where conspicuous; but as to its causes, they are mostly unknown, and must be resolved into original qualities of human nature, which I pretend not to explain.  Nothing is more requisite for a true philosopher, than to restrain the intemperate desire of searching into causes, and having established any doctrine upon a sufficient number of experiments, rest contented with that, when he sees a farther examination would lead him into obscure and uncertain speculations.  In that case his enquiry would be much better employed in examining the effects than the causes of his principle.

Amongst the effects of this union or association of ideas, there are none more remarkable, than those complex ideas, which are the common subjects of our thoughts and reasoning, and generally arise from some principle of union among our simple ideas.  These complex ideas may be divided into Relations, Modes, and Substances.  We shall briefly examine each of these in order, and shall subjoin some considerations concerning our general and particular ideas, before we leave the present subject, which may be considered as the elements of this philosophy.

SECT.  V. OF RELATIONS.

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.