makes it the business
of his life to get back to ultimate first principles must plainly be a
logician, though he need not be a specialist in biology or economics or
'sociology'. One great advantage which our children should have over
their parents as students of Philosophy is that the last half-century
has been one of unprecedented advance in the study of logic. In the
'logic of relations', founded by De Morgan, carried out further in the
third volume of Ernst Schroeder's _Algebra der Logik_, and made still
more precise in the earliest sections of the _Principia Mathematica_ of
Whitehead and Russell, we now possess the most potent weapon of
intellectual analysis ever yet devised by man.

We must further remark that the serious pursuit of any kind of science
implies not only that there _are_ truths, but that some of them, at
least, can be _known_ by man. Hence there arises a problem which is not
quite the same as that of logic. What _is_ the relation we mean to speak
of when we talk of 'knowing' something, and what conditions must be
fulfilled in order that a proposition may not only be true but be known
by us to be true? The very generality of this problem marks it out as
one which belongs to what I have been all along calling Philosophy. (We
must be careful to note that the problem does not belong to the 'special
science' of psychology. Psychology aims at telling us how particular
thoughts and trains of thought arise in an individual mind, but it has
nothing to say on the question which of our thoughts give us 'knowledge'
and which do not. The 'possibility of knowledge' has to be presupposed
by the psychologist as a pre-condition of his particular investigations
exactly as it is presupposed by the physicist, the botanist, or the
economist.) The study of the problem 'what are the conditions which must
be satisfied whenever anything at all is known' is precisely what Kant
meant by _Criticism_, though the raising of the problem in this
definite form is not due to Kant but goes back to Plato, who made it the
subject of one of his greatest dialogues, the _Theaetetus_. The simplest
way to make the nature and importance of the problem clear is perhaps
the way Mr. Russell adopts in the _Problems of Philosophy_--to give a
very rough statement of Kant's famous solution.

Kant held that careful analysis shows us that any piece of knowledge has
two constituents of very diverse origin. It has a _matter_ or material
constituent con