The second thesis of Gorgias was that, even granting the world to exist, it could not possibly be known. Here the reasoning is unexpectedly weak. Because all thoughts do not represent facts,—as, for example, our ideas of impossible combinations, like chariots running over the sea,—it is assumed that none do. But the problem how to distinguish between true and false ideas was raised, and it was round this that the fiercest battle between dogmatists and sceptics subsequently raged. And in the complete convertibility of consciousness and reality postulated by Gorgias, we may find the suggestion of a point sometimes overlooked in the automatist controversy—namely, that the impossibility, if any, of our acting on the material world reciprocally involves the impossibility of its acting on us, in so far as we are conscious beings. If thought cannot be translated into movement, neither can movement be translated into thought. The third thesis maintains that, granting the world to exist and to be knowable, one man cannot communicate his knowledge to another; for, the different classes of sensations being heterogeneous, a visual or tactual impression on our consciousness cannot be conveyed by an auditory impression on the consciousness of someone else. This difficulty has been completely overcome by the subsequent progress of thought. We cannot, it is true, directly communicate more than a few sensations to one another; but by producing one we may call up others with which it has become associated through previous experience. And the great bulk of our knowledge has been analysed into relations of co-existence, succession, and resemblance, which are quite independent of the particular symbols employed to transmit them from one mind to another.[220]