But the mischief lieth here; that when men of less leisure see them who are supposed to have spent their whole time in the pursuit of knowledge professing an entire ignorance of all things, or advancing such notions as are repugnant to plain and commonly received principles, they will be tempted to entertain suspicions concerning the most important truths which they had hitherto held sacred and unquestionable.--Berkeley's _Hylas and Philonous_. In no department of science is it possible for an enquirer to advance considerably beyond all his predecessors without serving as a light by whose aid his successors may advance somewhat beyond him. This is the only apology that I feel disposed to offer for the freedom with which I am about to criticize one who, having been, by judges so competent as Adam Smith and Professor Huxley, pronounced to be 'by far the greatest philosopher' and 'acutest thinker' of his own age, would, doubtless, be at least on a level with the greatest philosophers of the present age if he were living now. The veriest cripple that can manage to sit on horseback may contrive to crawl some few steps beyond the utmost point to which his steed has borne him, and, if those steps be uphill, may, by looking back on the course he has come, perceive where the animal has deviated from the right road. Yet he does not on that account suppose that his own locomotive power is in any respect to be compared to his horse's; neither need an annotator on Hume, when pointing to holes in his author's metaphysical coat, be supposed not to be perfectly aware that it is the strength, not of his own eyes, but of the spectacles furnished to him by his author, that enables him to perceive them. The concentrated essence of Hume's metaphysics is to be found in 'An Enquiry concerning Human Understanding,' forming part of a volume of Essays which Hume published somewhat late in life, and which he desired might 'alone be regarded as containing his philosophical sentiments and principles.' To a formal, though necessarily rapid, examination of the results of this 'Enquiry,' the present chapter will be almost exclusively devoted. Often as the operation has been performed already, there are two reasons why its repetition here may not be without utility: for, first, its subject is a treatise containing the germs of much subsequent and still current speculation which, in so far as it is merely a development of those germs, cannot but be infected by whatever unsoundness may be inherent in them; and, secondly, because the subject, hackneyed as it may seem, is so far from being exhausted, that there is scarcely one among the doctrines embodied in it to which, as I proceed at once to show, fresh objection, more or less grave, may not be taken by a fresh investigator.