Thus we can quite agree with Zeller when he observes[435] that Neo-Platonism only carried out a tendency towards spiritualism which had been already manifesting itself among the later Stoics, and had been still further developed by the Neo-Pythagoreans. But what does this prove? Not what Zeller contends for, which is that Neo-Platonism stands on the same ground with the other post-Aristotelian systems, but simply that a recurrence of the same intellectual conditions was being followed by a recurrence of the same results. Now, as before, materialism was proving its inadequacy to account for the facts of mental experience. Now, as before, morality, after being cut off from physical laws, was seeking a basis in religious or metaphysical ideas. Now, as before, the study of thoughts was succeeding to the study of words, and the methods of popular persuasion were giving place to the methods of dialectical demonstration. Of course, the age of Plotinus was far inferior to the age of Plato in vitality, in genius, and in general enlightenment, notwithstanding the enormous extension which Roman conquest had given to the superficial area of civilisation, as the difference between the _Enneads_ and the _Dialogues_ would alone suffice to prove. But this does not alter the fact that the general direction of their movement proceeds in parallel lines. In saying that the post-Aristotelian philosophers were not original thinkers, we must guard against the supposition that they contributed nothing of value to thought. On the contrary, while not putting forward any new theories, they generalised some of the principles borrowed from their predecessors, worked out others in minute detail, and stated the arguments on both sides of every controverted point with superior dialectic precision. Thus, while materialism had been assumed as self-evidently true by the pre-Socratic schools, it was maintained by the Stoics and Epicureans on what seemed to be grounds of experience and reason. And, similarly, we find that Plotinus, having arrived at the consciousness that spiritualism is the common ground on which Plato and Aristotle stand, the connecting trait which most completely distinguishes them from their successors, proceeds in his second essay[436] to argue the case against materialism more powerfully than it had ever been argued before, and with nearly as much effect as it has ever been argued since. V.