5. When any two objects possess the same _quality_ in common, the _degrees_ in which they possess it form a fifth species of relation. Thus, of two objects which are both heavy, the one may be either of greater or less weight than the other. Two colours, that are of the same kind, may yet be of different shades, and in that respect admit of comparison. 6. The relation of _contrariety_ may at first sight be regarded as an exception to the rule, _that no relation of any kind can subsist without some degree of resemblance_. But let us consider, that no two ideas are in themselves contrary, except those of existence and non-existence, which are plainly resembling, as implying both of them an idea of the object; though the latter excludes the object from all times and places, in which it is supposed not to exist. 7. All other objects, such as fire and water, heat and cold, are only found to be contrary from experience, and from the contrariety of their _causes_ or _effects_; which relation of cause and effect is a seventh philosophical relation, as well as a natural one. The resemblance implied in this relation shall be explained afterwards. It might naturally be expected that I should join _difference_ to the other relations; but that I consider rather as a negation of relation than as any thing real or positive. Difference is of two kinds, as opposed either to identity or resemblance. The first is called a difference of _number_; the other of _kind_. SECTION VI. OF MODES AND SUBSTANCES. I would fain ask those philosophers, who found so much of their reasonings on the distinction of substance and accident, and imagine we have clear ideas of each, whether the idea of _substance_ be derived from the impressions of sensation or reflection? If it be conveyed to us by our senses, I ask, which of them, and after what manner? If it be perceived by the eyes, it must be a colour; if by the ears, a sound; if by the palate, a taste; and so of the other senses. But I believe none will assert, that substance is either a colour, or sound, or a taste. The idea of substance must therefore be derived from an impression of reflection, if it really exist. But the impressions of reflection resolve themselves into our passions and emotions; none of which can possibly represent a substance. We have, therefore, no idea of substance, distinct from that of a collection of particular qualities, nor have we any other meaning when we either talk or reason concerning it.