M. Cousin, having apparently no conception of a way of acting so proper to legitimate inquiry, and having himself written a Course of Philosophy, evidently thinks Locke ought to have done the same; for he says that Locke is erroneous in the method of his philosophy, that he begins at the wrong end, that instead of having told us as he has how the ideas arise in the mind, he ought to have told us what the ideas are, instead of describing their origin to have described their actuality, to have given a list of the faculties of the mind, and so on. Which is just the same thing as saying that a traveller who publishes his explorations in America, ought instead to have gone to China. I shall have to make some objections to Locke, but they will be of a nature exactly contrary to those of which he is usually made the subject. Instead of accusing his principles I shall have to impute to him the not sufficiently carrying them out; a fault due to his position as an early reformer, and perfectly consistent with his high character as such. I have the more reason to note this distinction between M. Cousin's department and the function exercised by Locke, because I am forced myself to take the benefit of it. Want of erudition would form very vulnerable points, were I to be judged by the former standard. In the little I have yet put forth on the subject of First Principles, I already find two or three errors of that sort, which a greater amount of reading would no doubt have enabled me to escape. My present letter may close with some correction of one of these. Preliminary, I will venture to call "That which is is," a first principle of reason, and "Two and two make four," one of its derivatives, leaving this topic for future explanation, and then proceed thus:--When in my last letter I represented first principles as bounding the horizon of human knowledge, I left it to be inferred that both the kinds of "first principles" I had mentioned were thus describable in common. I find, however, that this metaphysical character belongs exclusively to first principles of sensuous experience, and no more belongs to first principles of reason than to first principles of grammar, or to first principles of rhetoric. That is, first principles of reason are merely the result of one of those analytical inquiries in which we arrive at something absolutely simple, and must there stop, just as in the science of numbers we may thus arrive at unity. Having long ago defined First Principles of sensuous experience, I find there is a difficulty attached to the other kind of first principles derived from the various use of the word reason--which I will say betrayed me into a wrong inference in the concluding paragraph of my last letter.