There are seven different kinds of philosophical relation,[1] viz. _resemblance, identity, relations of time and place, proportion in quantity or number, degrees in any quality, contrariety, and causation_. These relations may be divided into two classes; into such as depend entirely on the ideas, which we compare together, and such as may be changed without any change in the ideas. 'Tis from the idea of a triangle, that we discover the relation of equality, which its three angles bear to two right ones; and this relation is invariable, as long as our idea remains the same. On the contrary, the relations of _contiguity_ and _distance_ betwixt two objects may be changed merely by an alteration of their place, without any change on the objects themselves or on their ideas; and the place depends on a hundred different accidents, which cannot be foreseen by the mind. 'Tis the same case with _identity_ and _causation_. Two objects, though perfectly resembling each other, and even appearing in the same place at different times, may be numerically different: and as the power, by which one object produces another, is never discoverable merely from their idea, 'tis evident _cause_ and _effect_ are relations, of which we receive information from experience, and not from any abstract reasoning or reflection. There is no single phenomenon, even the most simple, which can be accounted for from the qualities of the objects, as they appear to us; or which we could foresee without the help of our memory and experience. It appears therefore that of these seven philosophical relations, there remain only four, which depending solely upon ideas, can be the objects of knowledge and certainty. These four are _resemblance, contrariety, degrees in quality, and proportions in quantity or number_. Three of these relations are discoverable at first sight, and fall more properly under the province of intuition than demonstration. When any objects _resemble_ each other, the resemblance will at first strike the eye, or rather the mind; and seldom requires a second examination. The case is the same with _contrariety_, and with the _degrees_ of any _quality_. No one can once doubt but existence and non-existence destroy each other, and are perfectly incompatible and contrary. And though it be impossible to judge exactly of the degrees of any quality, such as colour, taste, heat, cold, when the difference betwixt them is very small; yet 'tis easy to decide, that any of them is superior or inferior to another, when their difference is considerable. And this decision we always pronounce at first sight, without any inquiry or reasoning.