Chrysippus, had in the formation of the doctrines of the
school. But after Chrysippus the main lines of the doctrine were
complete. {345} We shall deal, therefore, with Stoicism as a whole,
and not with the special teaching of particular Stoics. The system is
divided into three parts, Logic, Physics, and Ethics, of which the
first two are entirely subservient to the last. Stoicism is
essentially a system of ethics which, however, is guided by a logic as
theory of method, and rests upon physics as foundation.

Logic.

We may pass over the formal logic of the Stoics, which is, in all
essentials, the logic of Aristotle. To this, however, they added a
theory, peculiar to themselves, of the origin of knowledge and the
criterion of truth. All knowledge, they said, enters the mind through
the senses. The mind is a _tabula rasa_, upon which sense-impressions
are inscribed. It may have a certain activity of its own, but this
activity is confined exclusively to materials supplied by the physical
organs of sense. This theory stands, of course, in sheer opposition to
the idealism of Plato, for whom the mind alone was the source of
knowledge, the senses being the sources of all illusion and error. The
Stoics denied the metaphysical reality of concepts. Concepts are
merely ideas in the mind, abstracted from particulars, and have no
reality outside consciousness.

Since all knowledge is a knowledge of sense-objects, truth is simply
the correspondence of our impressions to things. How are we to know
whether our ideas are correct copies of things? How distinguish
between reality and imagination, dreams, or illusions? What is the
criterion of truth? It cannot lie in concepts, since these are of our
own making. Nothing is true save {346} sense-impressions, and
therefore the criterion of truth must lie in sensation itself. It
cannot be in thought, but must be in feeling. Real objects, said the
Stoics, produce in us an intense feeling, or conviction, of their
reality. The strength and vividness of the image distinguish these
real perceptions from a dream or fancy. Hence the sole criterion of
truth is this striking conviction, whereby the real forces itself upon
our consciousness, and will not be denied. The relapse into complete
subjectivity will here be noted. There is no universally grounded
criterion of truth. It is based, not on reason, but on feeling. All
depends on the subjective convictions of the individual.

Physics.