Let him explain, if he can, what he means by his own identity, or the identity of any thinking or living being, which at different times consists of a totally different set of material particles. Something there clearly is which confers personal identity and constitutes an individual: it is a property characteristic of every form of life, even the humblest; but it is not yet explained or understood, and it is no answer to assert gratuitously that there is some fundamental "substance" or material basis on which that identity depends, any more than it is an explanation to say that it depends upon a "soul." These are all forms of words. As Hume says, quoted by Huxley with approval in the work already cited, p. 194:— "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 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.).