together, then a third feeling might arise which is neither
succession nor similarity, but that which we call _co-existence_.

These feelings, or their contraries, are the foundation of everything
that we call a relation. They are no more capable of being described
than sensations are; and, as it appears to me, they are as little
susceptible of analysis into simpler elements. Like simple tastes and
smells, or feelings of pleasure and pain, they are ultimate irresolvable
facts of conscious experience; and, if we follow the principle of Hume's
nomenclature, they must be called _impressions of relation_. But it must
be remembered, that they differ from the other impressions, in requiring
the pre-existence of at least two of the latter. Though devoid of the
slightest resemblance to the other impressions, they are, in a manner,
generated by them. In fact, we may regard them as a kind of impressions
of impressions; or as the sensations of an inner sense, which takes
cognizance of the materials furnished to it by the outer senses.

Hume failed as completely as his predecessors had done to recognise the
elementary character of impressions of relation; and, when he discusses
relations, he falls into a chaos of confusion and self-contradiction.

In the _Treatise_, for example, (Book I., Sec. iv.) resemblance, contiguity
in time and space, and cause and effect, are said to be the "uniting
principles among ideas," "the bond of union" or "associating quality by
which one idea naturally introduces another." Hume affirms that--

"These qualities produce an association among ideas, and upon the
appearance of one idea naturally introduce another." They are "the
principles of union or cohesion among our simple ideas, and, in
the imagination, supply the place of that inseparable connection by
which they are united in our memory. Here is a kind of
_attraction_, which, in the mental world, will be found to have as
extraordinary effects as in the natural, and to show itself in as
many and as various forms. Its effects are everywhere conspicuous;
but, as to its causes they are mostly unknown, and must be resolved
into _original_ qualities of human nature, which I pretend not to
explain."--(I. p. 29.)

And at the end of this section Hume goes on to say--

"Amongst the effects of this union or association of ideas, there
are none more remarkable than those complex ideas which