Now, it will probably do no harm to philosophers more than to other men, to be impressed with a sense of modesty and a right appreciation of the necessary limitations of their enterprise. You have been trying to soar beyond the atmosphere, and you will make the better use of your wings when you learn that they won't support you in a vacuum. Your failure is not due to the want of aquiline powers of flight, but to the melancholy truth that even an eagle can't do much in an air-pump. Is not that a rather consoling reflection? But here the philosopher begins to be recalcitrant. You are not lowering my pretensions, he says, but attacking the power of man to attain truth upon any terms. All that is given to us in experience is the effect of underlying causes; if the causes vary the effects would vary; and, unless, therefore, you can get back to the cause, your knowledge must remain empirical and radically uncertain. Destroy all transcendental truths, and the phenomenal world itself becomes a mere shifting phantasmagoria, on which we can trace only coincidences and sequences, but are entirely unable to say that they will ever recur again. The argument, of course, raises the recollection of library upon library of controversy. I can only touch one point. Practically, we do not trouble ourselves about this difficulty. We are quite convinced that we know a great many things: we are sure that the sun will rise and set to-morrow; we have no doubt as to the properties of the ordinary objects, of trees and stones and steam-engines; every action of our lives implies a certain confidence in what is called the uniformity of nature; and it is plain enough that even if our knowledge be, in some sense, only a knowledge of probabilities, yet, from its effect upon conduct, it may be exactly the same as a knowledge of certainties. There may be an indefinite distance between the "necessary truth" that two and two make four and the empirical truth that a stone will fall; but if all the evidence attainable goes to prove that the stone will fall, I should be as foolish not to act upon that hypothesis as not to assume the truth of the arithmetical formula. Now, it is, of course, the growth within recent generations of vast systems of such truths which has alarmed the philosopher. He contrasts his own fluctuating and conflicting dogmas with the steady growth and assured results and mutual confirmation of the established physical sciences. He fears that they will obtain a prestige which will enable them to crush him and sweep his pretended knowledge into the limbo of alchemy and astrology and scholastic logomachy. Here comes in the argument which is really the keystone of Mr. Balfour's whole theory; and, as I cannot accept it, I must dwell upon its true nature. It looks, at first sight, like a retort upon the men of science. Your knowledge, he seems to say, is as vain as your antagonist's. Your physics, and astronomy, and chemistry, and physiology