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 δόξα, 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 Experience were composed solely of sensations, each individual would be an isolated solipsistic unit--incapable of rational Discourse or communication with his fellow-men. To cure this defect, Plato offered the ideas--universal forms common to the intelligence of every rational being. Not only would they render possible a common Knowledge of Reality--the existence of such ideas would necessarily also give permanence, fixity, law, and order to our intellectual activity. Our Knowledge would not be a mere random succession of impressions, but a definitely determined organic unity.