aphysics._--Metaphysical dualism postulates the eternal
coexistence of mind and matter, as opposed to monism both idealistic and
materialistic. Two forms of this dualism are held. On the one hand it is
said that mind and matter are absolutely heterogeneous, and, therefore,
that any causal relation between them is _ex hypothesi_ impossible. On
the other hand is a hypothetical dualism, according to which it is held
that mind cannot bridge over the chasm so far as to _know_ matter _in
itself_, though it is compelled by its own laws of cause and effect to
postulate matter as the origin, if not the motive cause, of its
sensations. It follows that, for the thinking mind, matter is a
necessary hypothesis. Hence the theory is a kind of monism, inasmuch as
it confessedly does not assert the existence of matter save as an
intellectual postulate for the thinking mind. Matter, in other words,
must be assumed to exist, though mind cannot know it _in itself_. From
this question there emerges a second and more difficult problem.
Consciousness, it is held, is of two main kinds, sensation and reason.
Sensation alone is insufficient to explain all our intellectual
phenomena; all sensation is momentary and individual (cf. Empiricism).
How then are we to account for memory and the principles of necessity,
similarity, universality? It is argued that there must be in the mind an
enduring, primary faculty whereby we retain, compare and group the
presentations of sense. This faculty is _a priori_, transcendental, and
entirely separate from all the data of experience and sense-perception.
Here then we have a dualism within experience. The mind is not to be
regarded as a sensitized film which automatically records the
impressions of the senses. It contains within itself this modifying
critical faculty which reacts upon and arranges the sense-given
presentations.

_In Ethics and Theology._--In the domain of morals, dualism postulates
the separate existence of Good and Evil, as principles of existence. In
theology the appearance of dualism is sporadic and has not the
fundamental, determining importance which it has in metaphysics. It is a
result rather than a starting-point. The old Zoroastrianism, and those
Christian sects (e.g. Manichaeism) which were influenced by it,
postulate two contending deities Ormuzd and Ahriman (Good and Evil),
which war against one another in influencing the conduct of men. So, in
Christianity, the existence of Satan a