, 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