Natural relation

• resemblance, contiguity, and the causal relation

The natural relations are the qualities “by which two ideas are connected together in the imagination, and the one naturally introduces the other, after the manner above explained” [T/1/1/5]. These qualities are resemblance, contiguity, and the causal relation. In these, ideas are connected with one another by either a natural or habitual force of association, so that thinking the one tends to force to recollection of the other.

Philosophical relation

• resemblance, identity, relations of time and place (covering contiguity), proportion in quantity or number, degrees in any quality, contrariety, and causation

The philosophical relations, on the other hand, are not impelled by a natural force of association, but arise from the reflective comparison of objects. There are seven types of philosophical relation: resemblance, identity, relations of time and place (covering contiguity), proportion in quantity or number, degrees in any quality, contrariety, and causation. [T/1/1/5] The three natural relations appear in this list, but not in their capacity as natural (forceful). Resemblance, for example, is compulsory only if sufficiently specific. A very general quality, however, does not goad the mind from one member of class (say, of red things) to another by the force of association. Likewise, spatiotemporal relations lack all associative force except what has been gained by habit. The proximity of two objects arbitrarily placed together is a reflective determination. But if I have always experienced two things as spatially and immediately contiguous, or as temporally successive, I will by compelled to associate them in this way and so believe in the reality of objective substance and causality.

Philosophical relations (with also-Natural in bold)

• identity, resemblance, contrariety
• proportion in quantity or number
• degrees in any quality
• relations of time and place (contiguity)
• causation