Modes are conditions which do not subsist of themselves, but have need of a basis or support; they are not conceivable apart from a thing whose properties or states they are. We notice that certain qualities always appear together, and habitually refer them to a substratum as the ground of their unity; in which they subsist or from which they proceed. _Substance_ denotes this self-existent "we know not what," which has or bears the attributes in itself, and which arouses the ideas of them in us. It is the combination of a number of simple ideas which are presumed to belong to one thing. From the ideas of sensation the understanding composes the idea of body, and from the ideas of reflection that of mind. Each of these is just as clear and just as obscure as the other; of each we know only its effects and its sensuous properties; its essence is for us entirely unknowable. Instead of the customary names, material and immaterial substances, Locke recommends cogitative and incogitative substances, since it is not inconceivable that the Creator may have endowed some material beings with the capacity of thought. God,--the idea of whom is attained by uniting the ideas of existence, power, might, knowledge, and happiness with that of infinity,--is absolutely immaterial, because not passive, while finite spirits (which are both active and passive) are perhaps only bodies which possess the power of thinking. While the ideas of substances are referred to a reality without the mind as their archetype, to which they are to conform and which they should image and represent, _Relations_ (_e.g._, husband, greater) are free and immanent products of the understanding. They are not copies of real things, but represent themselves alone, are their own archetypes. We do not ask whether they agree with things, but, conversely, whether things agree with them (Book iv. 4.5). The mind reaches an idea of relation by placing two things side by side and comparing them. If it perceives that a thing, or a quality, or an idea begins to exist through the operation of some other thing, it derives from this the idea of the causal relation, which is the most comprehensive of all relations, since all that is actual or possible can be brought under it. _Cause_ is that which makes another thing to begin to be; _effect_, that which had its beginning from some other thing. The production of a new quality is termed alteration; of artificial things, making; of a living being, generation; of a new particle of matter, creation. Next in importance is the relation of _identity and diversity_. Since it is impossible for a thing to be in two different places at the same time and for two things to be at the same time in the same place, everything that at a given instant is in a given place is identical with itself, and, on the other hand, distinct from everything else (no matter how great the resemblance between them) that at the same moment exists in another place.