Logic the Stoics divided into two parts—Rhetoric, the ‘science of the open
hand,’ and Dialectic, the ‘science of the closed fist,’ as Zeno called them.
They indulged in elaborate divisions and subdivisions of each, with which we
need not meddle. The only points of interest to us are contained in their
analysis of the processes of perception and thought. A sensation, Zeno
taught, was the result of an external impulse, which when combined
with an internal assent, produced a mental state that revealed at the
same time itself and the external object producing it. The perception thus
produced he compared to the grip which the hand took of a solid object; and
real perceptions, those, that is, which were caused by a real external
object, and not by some illusion, always testified to the reality of their
cause by this sensation of ‘grip.’
The internal assent of the mind was voluntary, and at the same time
necessary; for the mind could not do otherwise than will the acceptance of
that which it was fitted to receive. The peculiarity of their physics, which
we shall have to refer to later on, namely, the denial of the existence of
anything not material, implied that in some way there was a material action
of the external object on the structure of the perceiving mind (itself also
material). What exactly the nature of this action was the Stoics themselves
were not quite agreed. The idea of an ‘impression’ such as a seal makes upon
wax was a tempting one, but they had difficulty in comprehending how there
could be a multitude of different impressions on the same spot without
effacing each other. Some therefore preferred the vaguer and safer
expression, ‘modification’; had they possessed our modern science, they
might have illustrated their meaning by reference to the phenomena of
magnetism or electricity.
A History of Greek Philosophy /
THE STOICS
