of, several individuals. That can
never be true of sensation. We can never tell whether our sensations are
the same as those of other people--never at any rate by means of
sensations themselves; never unless and until such sensations have been
inter-related by some other instrument. A mere photographic reproduction
of sensation is thus quite useless as a means of Knowledge.

In some way or other general terms supply the common bond. The
recognition of this fact was one of the great results of the Socratic
discussion. This explains the immense importance which Socrates
naturally attached to the criticism of general and abstract terms.

* * * * *

The work of Socrates in this direction was immediately taken up and
carried much further by Plato. Plato maintained that these general and
abstract terms were in truth the names of ideas (+eide+) with which the
mind is naturally furnished, and further that these ideas corresponded
to and typified the eternal forms of things--the essential constituents
of the real world. Knowledge was possible because there were such
eternal forms or ideal elements--the archetypes--of which the +eide+
were the counterparts and representations.

Knowledge, Plato held, was concerned solely with these eternal forms,
not with sensation at all. The sensible world was in a state of constant
flux and could not be the object of true science. Its apprehension was
effected by a faculty or capacity (_Republic_, v. 478-79) midway between
Knowledge and nescience to which he applied the term +doxa+, frequently
translated _opinion_, but which in this connection would be much more
accurately rendered, _sensible impression_, or even perception. At any
rate, the term _opinion_ is a very unhappy one, and does not convey the
true meaning at all, for no voluntary intellective act on the part of
the subject was implied by the term. Now intelligence in constructing a
scheme of Knowledge is active. The ideas are the instruments of this
activity.

Plato's doctrine of ideas was probably designed or conceived by him as
affording an explanation also of the community of Knowledge. He
emphasised the fluent instability of the sensible impression, and as we
have already pointed out, sensation in itself labours also under this
drawback that it contains and affords no common nexus whereby the
conceptions or perceptions of one man can be compared or related with
those of another.

Indeed, if E