Again, the spiritualism taught by Plato and Aristotle alike—by the disciple, indeed, with even more distinctness than by the master—was so entirely inconsistent with the common belief of antiquity as to remain a dead letter for nearly six centuries—that is, until the time of Plotinus. The difference between body and mind was recognised by every school, but only as the difference between solid and gaseous matter is recognised by us; while the antithesis between conscious and unconscious existence, with all its momentous consequences, was recognised by none. The old hypothesis had to be thoroughly thought out before its insufficiency could be completely and irrevocably confessed. Nor was this the only reason why the spiritualists lost touch of their age. If in some respects they were far in advance of early Greek thought, in other respects they were far behind it. Their systems were pervaded by an unphilosophical dualism which tended to undo much that had been achieved by their less prejudiced predecessors. For this we have partly to blame their environment. The opposition of God and the world, heaven and earth, mind and matter, necessity in Nature and free-will in man, was a concession—though of course an unconscious concession—to the stupid bigotry of Athens. Yet at the same time they had failed to solve those psychological problems which had most interest for an Athenian public. Instead of following up the attempt made by the Sophists and Socrates to place morality on a scientific foundation, they busied themselves with the construction of a new machinery for diminishing the efficacy of temptation or for strengthening the efficacy of law. To the question, What is the highest good? Plato gave an answer which nobody could understand, and Aristotle an answer which was almost absolutely useless to anybody but himself. The other great problem, What is the ultimate foundation of knowledge? was left in an equally unsatisfactory state. Plato never answered it at all; Aristotle merely pointed out the negative conditions which must be fulfilled by its solution.