People who think—as against those who feel and act—want hard and fast lines—without which, indeed, they cannot think at all; these lines are as it were steps cut on a slope of ice without which there would be no descending it. When we have begun to travel the downward path of thought, we ask ourselves questions about life and death, ego and non ego, object and subject, necessity and free will, and other kindred subjects. We want to know where we are, and in the hope of simplifying matters, strip, as it were, each subject to the skin, and finding that even this has not freed it from all extraneous matter, flay it alive in the hope that if we grub down deep enough we shall come upon it in its pure unalloyed state free from all inconvenient complication through intermixture with anything alien to itself. Then, indeed, we can docket it, and pigeon-hole it for what it is; but what can we do with it till we have got it pure? We want to account for things, which means that we want to know to which of the various accounts opened in our mental ledger we ought to carry them—and how can we do this if we admit a phenomenon to be neither one thing nor the other, but to belong to half-a-dozen different accounts in proportions which often cannot even approximately be determined? If we are to keep accounts we must keep them in reasonable compass; and if keeping them within reasonable compass involves something of a Procrustean arrangement, we may regret it, but cannot help it; having set up as thinkers we have got to think, and must adhere to the only conditions under which thought is possible; life, therefore, must be life, all life, and nothing but life, and so with death, free will, necessity, design, and everything else. This, at least, is how philosophers must think concerning them in theory; in practice, however, not even John Stuart Mill himself could eliminate all taint of its opposite from any one of these things, any more than Lady Macbeth could clear her hand of blood; indeed, the more nearly we think we have succeeded the more certain are we to find ourselves ere long mocked and baffled; and this, I take it, is what our biologists began in the autumn of 1879 to discover had happened to themselves.