To the objection that his suspensive attitude would render action impossible, Arcesilaus replied that any mental representation was sufficient to set the will in motion; and that, in choosing between different courses, probability was the most rational means of determination. But the task of reducing probable evidence to a system was reserved for a still abler dialectician, who did not appear on the scene until a century after his time. Arcesilaus is commonly called the founder of the Middle, Carneades the founder of the New Academy. The distinction is, however, purely nominal. Carneades founded nothing. His principles were identical with those of his predecessor; and his claim to be considered the greatest of the Greek sceptics is due to his having given those principles a wider application and a more systematic development. The Stoics regarded it as a special dispensation of providence that Chrysippus, the organising genius of their school, should have come between its two most formidable opponents, being thus placed in a position to answer the objections of the one and to refute by anticipation those of the other.[232] It might seem to less prejudiced observers that the thinker whose cause benefited most by this arrangement was Carneades. Parodying a well-known iambic, he used to say: ‘Without Chrysippus I should not have been.’[233] And, in fact, it was by a close study of that writer’s voluminous treatises that he was able to cover the immense extent of ground which Scepticism thenceforward disputed with the dogmatic schools. Nor were his attacks directed against Stoicism only, but against all other positive systems past and present as well. What he says about the supposed foundation of knowledge is even now an unanswerable objection to the transcendental realism of Mr Herbert Spencer. States of consciousness speak for themselves alone, they do not include the consciousness of an external cause.[234] But the grounds on which he rests his negation of all certainty are still superficial enough, being merely those sensible illusions which the modern science of observation has been able either to eliminate altogether or to restrict within narrow and definable limits. That phenomena, so far from being necessarily referred to a cause which is not phenomenal, cannot be thought of at all except in relation to one another, and that knowledge means nothing more than a consciousness of this relation, was hardly perceived before the time of Hume.