"It is impossible to attach any definite meaning to the word 'substance,' when employed for the hypothetical substratum of soul and matter.... If it be said that our personal identity requires the assumption of a substance which remains the same while the accidents of perception shift and change, the question arises what is meant by personal identity?... A plant or an animal, in the course of its existence, from the condition of an egg or seed to the end of life, remains the same neither in form, nor in structure, nor in the matter of which it is composed: every attribute it possesses is constantly changing, and yet we say that it is always one and the same individual" (p. 194). And in his own preface to the Hume volume Huxley expresses himself forcibly thus--equally antagonistic as was his wont to both ostensible friend and ostensible foe, as soon as they got off what he considered the straight path:-- "That which it may be well for us not to forget is, that the first-recorded judicial murder of a scientific thinker [Socrates] was compassed and effected, not by a despot, nor by priests, but was brought about by eloquent demagogues.... Clear knowledge of what one does not know is just as important as knowing what one does know.... "The development of exact natural knowledge in all its vast range, from physics to history and criticism, is the consequence of the working out, in this province, of the resolution to 'take nothing for truth without clear knowledge that it is such'; to consider all beliefs open to criticism; to regard the value of authority as neither greater nor less, than as much as it can prove itself to be worth. The modern spirit is not the spirit 'which always denies,' delighting only in destruction; still less is it that which builds castles in the air rather than not construct; it is that spirit which works and will work 'without haste and without rest,' gathering harvest after harvest of truth into its barns, and devouring error with unquenchable fire" (p. viii). The harvesting of truth is a fairly safe operation, for if some falsehood be inadvertently harvested along with the grain we may hope that, having a less robust and hardy nature, it will before long be detected by its decaying odour; but the rooting up and devouring of error with unquenchable fire is a more dangerous enterprise, inasmuch as flames are apt to spread beyond our control; and the lack of infallibility in the selection of error may to future generations become painfully apparent. The phrase represents a good healthy energetic mood however, and in a world liable to become overgrown with weeds and choked with refuse, the cleansing work of a firebrand may from time to time be a necessity, in order that the free wind of heaven and the sunlight may once more reach the fertile soil.