The fundamental proposition of the Stoic physics is that "nothing
incorporeal exists." This materialism coheres with the sensationalism
of their doctrine of knowledge. Plato placed knowledge in thought, and
reality, therefore, in the Idea. The Stoics, however, place knowledge
in physical sensation, and reality, therefore, in what is known by the
senses, matter. All things, they said, even the soul, even God
himself, are material and nothing more than material. This belief they
based upon two main considerations. Firstly, the unity of the world
demands it. The world is one, and must issue from one principle. We
must have a monism. The idealism of Plato and Aristotle had resolved
itself into a futile struggle against the dualism of matter and
thought. Since the gulf cannot be bridged from the side of the Idea,
we must take our stand on matter, and reduce mind to it. Secondly,
body and soul, God and {347} the world, are pairs which act and react
upon one another. The body, for example, produces thoughts
(sense-impressions) in the soul, the soul produces movements in the
body. This would be impossible if both were not of the same substance.
The corporeal cannot act on the incorporeal, nor the incorporeal on
the corporeal. There is no point of contact. Hence all must be equally
corporeal.

All things being material, what is the original kind of matter, or
stuff, out of which the world is made? The Stoics turned to
Heracleitus for an answer. Fire is the primordial kind of being, and
all things are composed of fire. With this materialism the Stoics
combined pantheism. The primal fire is God. God is related to the
world exactly as the soul to the body. The human soul is likewise
fire, and comes from the divine fire. It permeates and penetrates the
entire body, and, in order that its interpenetration might be regarded
as complete, the Stoics denied the impenetrability of matter. Just as
the soul-fire permeates the whole body, so God, the primal fire,
pervades the entire world. He is the soul of the world. The world is
His body.

But in spite of this materialism, the Stoics averred that God is
absolute reason. This is not a return to idealism. It does not imply
the incorporeality of God. For reason, like all else, is material. It
means simply that the divine fire is a rational element. Since God is
reason, it follows that the world is governed by reason, and this
means two things. It means, firstly, that there is purpose in t