But if we consider it from one point of view the treatise on the _Conduct of the Understanding_ is itself a witness that human reason is a compass liable to incalculable variations and likely enough to shipwreck those who steer by it alone. In this book Locke shows us that to come to a true result the understanding (1) must be perfectly trained, (2) must not be affected by any feeling in favour of or against any particular result, and (3) must have before it all the data necessary for forming a judgment. In practice these conditions are seldom (if ever) fulfilled; and Locke himself, when he wants an instance of a mind that can acquiesce in the certainty of its conclusions, takes it from “angels and separate spirits who may be endowed with more comprehensive faculties” than we are (C. of U. § iij, 3). § 5. It seems to me then that Locke much exaggerates the power of the individual reason for getting at the truth. And to exaggerate the importance of one function of the mind is to unduly diminish the importance of the rest. Thus we find that in Locke’s scheme of education little thought is taken for the play of the affections and feelings; and as for the imagination it is treated merely as a source of mischief. § 6. Locke, as it has often been pointed out, differs from the schoolmaster in making small account of the knowledge to be acquired by those under education. But it has not been so often remarked that the fundamental difference is much deeper than this and lies in the conception of knowledge itself. With the ordinary schoolmaster the test of knowledge is the power of reproduction. Whatever pupils can reproduce with difficulty they know imperfectly; whatever they can reproduce with ease they know thoroughly. But Locke’s definition of knowledge confines it to a much smaller area. According to him knowledge is “the internal perception of the mind” (Locke to Stillingfleet _v._ F. B. ij, 432). “Knowing is seeing; and if it be so, it is madness to persuade ourselves we do so by another man’s eyes, let him use never so many words to tell us that what he asserts is very visible. Till we ourselves see it with our own eyes, and perceive it by our own understandings, we are as much in the dark and as void of knowledge as before, let us believe any learned authors as much as we will” (C. of U. § 24).[113] § 7. Here Locke makes no distinction between different classes of truths. But surely very important differences exist. About some physical facts our knowledge is at once most certain and most definite when we derive it through the evidence of our own senses. “Seeing is believing,” says the proverb. It may be believing, but it is not knowing. That certainty which we call knowledge we often arrive at better by the testimony of others than by that of our own senses.