"The key to all philosophy lies in the clear apprehension of Berkeley's problem--which is neither more nor less than one of the shapes of the greatest of all questions, 'What are the limits of our faculties?' And it is worth any amount of trouble to comprehend the exact nature of the argument by which Berkeley arrived at his results, and to know by one's own knowledge the great truth which he discovered--that the honest and rigorous following up of the argument which leads us to materialism inevitably carries us beyond it." Huxley, however, while he opposed a materialistic explanation of the universe with the strength of exposition and acute reasoning at his disposal, did not pass directly into the other camp and become a pure idealist. "Granting the premisses," he wrote, "I do not see any escape from Berkeley's conclusion, that the substance of matter is a metaphysical unknown quantity, of the existence of which there is no proof. What Berkeley does not seem to have so clearly perceived is that the non-existence of a substance of mind is equally arguable; and that the result of the impartial application of his reasonings is the reduction of the all to co-existences and sequences of phenomena, beneath and beyond which there is nothing cognoscible." Hume had written: "What we call a mind is nothing but a heap or collection of different perceptions, united together by certain relations, and supposed, though falsely, to be endowed with a perfect simplicity and identity." Here was mind rejected for the same negative reasons as matter, and Huxley was as ready to point out that while we can know nothing of the "substance of the thinking thing, we go beyond legitimate reasoning if we therefore deny its existence." ... "Hume may be right or wrong, but the most he or anyone else can prove in favour of his conclusions is, that we know nothing more of the mind than that it is a series of perceptions. Whether there is something in the mind that lies beyond the reach of observation, or whether perceptions themselves are the products of something which can be observed and which is not mind, are questions which can in no wise be settled by direct observation." In another passage he writes: