In all this argument it must be remembered Plato never said or suggested that the intellect of man—thus equipped with ideal forms—was thereby enabled to become, or did become, the creator of the world by and in which each one believes himself to be surrounded and included. He always distinguished between Idea and Reality, between Thought and Thing. The ideas were types of the forms immanent in things themselves. It has been said by some scholars that he generally distinguished between the two by the employment of distinct terms, applying εἷδος to the mental conception and ἰδέα to the substantial form. This verbal distinction was accepted by many scholars of the epoch of Liddell and Scott and Davies and Vaughan. A reference to this distinction in the present writer's essay on The Dynamic Foundation of Knowledge provoked at the instance of one critic the allegation that it is not borne out by a critical study of the Platonic texts. That is a matter of little moment and one upon which the writer cannot claim to pronounce. The important point is that in one way or another Plato undoubtedly distinguished between and indeed contrasted the idea and the substantial form. No trace of the solipsism which results from their being confounded and which has ultimately brought to destruction the imposing edifice of Hegelian Thought is to be found in his writings.
The Platonic doctrine of ideas speedily found an energetic critic in Aristotle. In Aristotle's view, it was quite unnecessary and unwarrantable to postulate the existence in the Mind of ideal forms or counterparts of the substantial forms of Reality. This, according to him, was a wholly unnecessary reduplication. He was content to believe that the mind found and recognised the essential forms of things when they were presented to it in perceptive Experience. Universalia in re were conceived by him as sufficiently explaining the genesis of cognition without the postulation of any such universalia extra rem.
To the Platonic doctrine he offered the further objection that the eternal forms of things which that doctrine affirmed and which it declared to be represented in their ideal types were necessarily impotential. There was no generative power in the pure activity of Thought. If, therefore, the essentials of Reality were ideal, it followed that they also were impotent, and incapable of causative efficacy. The sensible world, however, was a fluent and perpetually generated stream, which required some potent cause to uphold it.
The eternal Reality which sustained the world was for him an Energy constantly generating the actual, and no conception which failed to provide for this process of causative generation of the things of Sense could in his view adequately account for the phenomena of Nature nor consequently could constitute the system of science.
In this argument Aristotle undoubtedly expressed a profound truth, but it may perhaps be admitted that he rather failed to appreciate fully the difficulty which the Platonic doctrine was designed to meet—that, namely, of providing some sort of common nexus or unifying principle by which the validity of Knowledge could be maintained. For he had no certain means of showing that the potent energy of Nature was unitary and homogeneous.
He is frequently described as a sensationalist, but such a view is certainly incorrect. This, however, may be admitted—that he sought the essentials of Reality not in the Mind but in the Object. It may be fairly claimed that to this extent he occupied common ground with the sensationalists, in that he was an adherent of the tabula rasa view of the Mind, expressed in the maxim:—
Nihil est in intellectu quod non fuit in sensu.