(a) In the age of Socrates the mind was passing from the object to the subject. The same impulse which a century before had led men to form conceptions of the world, now led them to frame general notions of the human faculties and feelings, such as memory, opinion, and the like. The simplest of these is sensation, or sensible perception, by which Plato seems to mean the generalized notion of feelings and impressions of sense, without determining whether they are conscious or not. The theory that 'Knowledge is sensible perception' is the antithesis of that which derives knowledge from the mind (Theaet.), or which assumes the existence of ideas independent of the mind (Parm.). Yet from their extreme abstraction these theories do not represent the opposite poles of thought in the same way that the corresponding differences would in modern philosophy. The most ideal and the most sensational have a tendency to pass into one another; Heracleitus, like his great successor Hegel, has both aspects. The Eleatic isolation of Being and the Megarian or Cynic isolation of individuals are placed in the same class by Plato (Soph.); and the same principle which is the symbol of motion to one mind is the symbol of rest to another. The Atomists, who are sometimes regarded as the Materialists of Plato, denied the reality of sensation. And in the ancient as well as the modern world there were reactions from theory to experience, from ideas to sense. This is a point of view from which the philosophy of sensation presented great attraction to the ancient thinker. Amid the conflict of ideas and the variety of opinions, the impression of sense remained certain and uniform. Hardness, softness, cold, heat, etc. are not absolutely the same to different persons, but the art of measuring could at any rate reduce them all to definite natures (Republic). Thus the doctrine that knowledge is perception supplies or seems to supply a firm standing ground. Like the other notions of the earlier Greek philosophy, it was held in a very simple way, without much basis of reasoning, and without suggesting the questions which naturally arise in our own minds on the same subject.