of
each, with which we need not meddle. The only points of interest to us
are contained in their analysis [392] 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.'

{231}

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.

An interesting passage may be quoted from [393] Plutarch on the Stoic
doctrine of knowledge: "The Stoics maintain," he says, "that when a
human being is born, he has the governing part of his soul like a sheet
of paper ready prepared for the reception of writing, and on this the
soul inscribes in succession its various ideas. The first form of the
writing is produced through the senses. When we perceive, for example,
{232} a white object, the recollection remains when the object is gone.
And when many similar recollections have accumulated, we have what is
called _experience_. Besides the ideas which we get in this natural
and quite undesigned way, there are other ideas which we get through
teaching and information. In the strict sense only these latter ought
to be called ideas; the former should rat