Locke, in his "Essay concerning Human Understanding," undertakes "to inquire into the original, certainty, and extent of human knowledge, together with the grounds and degrees of belief, opinion, and assent." His sober and cautious work, which was first published in 1690, was peculiarly English in character; and the spirit which it exemplifies animates also Locke's famous successors, George Berkeley (1684-1753), David Hume (1711-1776), and John Stuart Mill (1806-1873). Although Locke was a realist, Berkeley an idealist, Hume a skeptic, and Mill what has been called a sensationalist; yet all were empiricists of a sort, and emphasized the necessity of founding our knowledge upon experience. Now, Locke was familiar with the writings of Descartes, whose work he admired, but whose rationalism offended him. The first book of the "Essay" is devoted to the proof that there are in the mind of man no "innate ideas" and no "innate principles." That is to say, Locke tries to show that one must not seek, in the "natural light" to which Descartes turned, a distinct and independent source of information, "Let us, then," he continues, "suppose the mind to be, as we say, white paper, void of all characters, without any ideas; how comes it to be furnished? Whence comes it by that vast store which the busy and boundless fancy of man has painted on it, with an almost endless variety? Whence has it all the materials of reason and knowledge? To this I answer in one word, from experience; in that all our knowledge is founded, and from that it ultimately derives itself. Our observation, employed either about external sensible objects, or about the internal operations of our minds, perceived and reflected on by ourselves, is that which supplies our understandings with all the materials of thinking. These two are the fountains of knowledge, from whence all the ideas we have, or can naturally have, do spring." [1] Thus, all we know and all we ever shall know of the world of matter and of minds must rest ultimately upon observation,--observation of external things and of our own mind. We must clip the erratic wing of a "reason" which seeks to soar beyond such knowledge; which leaves the solid earth, and hangs suspended in the void. "But hold," exclaims the critical reader; "have we not seen that Locke, as well as Descartes (section 48), claims to know what he cannot prove by direct observation or even by a legitimate inference from what has been directly observed? Does he not maintain that the mind has an immediate knowledge or experience only of its own ideas? How can he prove that there are material extended things outside causing these ideas? And if he cannot prove it by an appeal to experience, to direct observation, is he not, in accepting the existence of the external world at all, just as truly as Descartes, a rationalist?"