crisis. But the great liberating and suggestive value of Berkeley's principle remains: we are forced to recognize that the world cannot be formed of a substance different from that of our own thought. How, indeed, can we say that we know the world if we admit the possibility of knowing something foreign to thought, something which is not thought? From this principle, through Hume, the great reversal of Kant and all German idealism down to Hegel are derived; human thought henceforth, despite all the possible stupidities of science, cannot go back beyond this point.
Berkeley himself, it is true, did not maintain his principle in absolute purity to the end. In the Siris, the work of his old age, though he remains a spiritualist, Plato has led him toward the more naturalistic idealism of the Greeks. His ideas are no longer those of the Principles, they are those of Plato; and between the Supreme Spirit and the spirits of men there intervenes the universal fire or ether, which displays the chemical and biological phenomena of the universe, and can scarcely be reduced to spirit, though conceived as a divine emanation.
But men will forget the erudite neo-Platonism of Berkeley's old age, and will remember the immaterialism of his youth. For that theory, though expressed in empiric language by a posi-
