Sir G. G. Stokes begins by promising to confine himself to the question, "What is it that personal identity depends upon and consists in?" But he does not fulfil the promise. After some jejune remarks upon this question he drops into theology and winds up with a little sermon. "I cannot pretend that I am able to answer that question myself," says Sir G. G. Stokes. Why, then, did he not leave it alone? "But I will endeavor," he says, "to place before you some thoughts bearing in that direction which I have found helpful to myself, and which possibly may be of some help to some of you." Sir G. G. Stokes does not mention David Hume, but that great thinker pointed out, with his habitual force and clearness, that personal identity depends upon memory. Our scientific lecturer, with the theological twist, says it "involves memory," which implies a certain reservation. Yet he abstains from elucidating the point; and as it is the most important one in the discussion, he must be held guilty of short-sightedness or timidity. Memory involves thought, says Sir G. G. Stokes. This is true; in fact, it is a truism. And what, he asks, does thought depend on? "To a certain extent" he allows that it "depends upon the condition of the brain." But during the present life, at any rate, it depends _absolutely_ on the condition of the brain Look at the head of an idiot, and then at the head of Shakespeare; is not the brain difference the obvious cause of the mental difference? Are there not diseases of the brain that affect thought in a definite manner? Is not thought excited by stimulants, and deadened or even annihilated by narcotics? Is it not entirely suspended in healthy sleep? Will not a man of genius become an imbecile if his brain softens? Will not a philosopher rave like a drunken fishfag if he suffers from brain inflammation? Is not thought most vigorous when the brain is mature? And is it not weakest in the first and second childishness of youth and old age? The dependence of thought on the brain is so obvious, it is so demonstrable by the logical methods of difference and concomitant variations, that whoever disputes it, or only allows it "to a certain extent," is bound to assign another definite cause. A _definite_ cause, we say; not a fanciful or speculative one, which is perfectly hypothetical. Sir G. G. Stokes does not do this. He tries to make good his reservation by a negative criticism of "the materialistic hypothesis." He takes the case of a man who, while going up a ladder and speaking, was knocked on the head by a falling brickbat. For two days he was unconscious, and "when he came to, he completed the sentence that he had been speaking when he was struck." Now, at first sight, this seems a strong confirmation of "the materialistic hypothesis." A shock to the brain stopped its action and suspended consciousness. Automatic animal functions went on, but there was no perception, thought, or feeling.