Both defects are closely connected. The Greek philosophers were content with an uncertain and imperfect knowledge of facts, because they trusted conceptions too implicitly, and were ignorant of their origin and worth; and they had this unconditional trust in the truth of conceptions because the study of nature was yet in its infancy. Their knowledge of history was too limited for them to see the difference between the results of careful observation and those of ordinary unmethodical experience, to realise the uncertainty of most of the traditional principles and the necessity for a stricter method of induction. The fault common to both Plato and Aristotle lay in attaching undue prominence to the dialectical method inherited from Socrates to the neglect of observation, and in assuming that conceptions expressing the very essence of things can be deduced in a purely logical way from current beliefs and the use of language. In Plato this dialectical exclusiveness appears most strongly, and finds striking expression in his theory of recollection. If all conceptions are inherent from the moment of birth and need only the agency of sensible things to produce a consciousness of their existence, it is only legitimate to infer that, to know the essence of things, we must look within and not without, and obtain ideas by abstraction from the mind rather than by induction from experience. It is equally legitimate to infer that the ideas derived from the mind are the true standard by which experience must be judged. Whenever ideas and experience disagree, instead of regarding ideas as at fault, we ought to look upon the data of experience as imperfect, and as inadequately expressing the ideas which constitute the thing as it really exists. Thus the whole theory of ideas, and all that it implies, is seen to be a natural corollary from the Socratic theory of conceptions. Even those parts of this theory which seem most incongruous are best explained by being referred to the principles of the Socratic process.