ce 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 diffe