tum. Thus in
regard to physical objects, for example, the principle that sense-data
are signs of physical objects is itself a connexion of universals; and
it is only in virtue of this principle that experience enables us to
acquire knowledge concerning physical objects. The same applies to
the law of causality, or, to descend to what is less general, to such
principles as the law of gravitation.

Principles such as the law of gravitation are proved, or rather are
rendered highly probable, by a combination of experience with some
wholly _a priori_ principle, such as the principle of induction. Thus
our intuitive knowledge, which is the source of all our other knowledge
of truths, is of two sorts: pure empirical knowledge, which tells us of
the existence and some of the properties of particular things with
which we are acquainted, and pure _a priori_ knowledge, which gives us
connexions between universals, and enables us to draw inferences from
the particular facts given in empirical knowledge. Our derivative
knowledge always depends upon some pure _a priori_ knowledge and usually
also depends upon some pure empirical knowledge.

Philosophical knowledge, if what has been said above is true, does not
differ essentially from scientific knowledge; there is no special
source of wisdom which is open to philosophy but not to science, and the
results obtained by philosophy are not radically different from those
obtained from science. The essential characteristic of philosophy,
which makes it a study distinct from science, is criticism. It examines
critically the principles employed in science and in daily life; it
searches out any inconsistencies there may be in these principles,
and it only accepts them when, as the result of a critical inquiry, no
reason for rejecting them has appeared. If, as many philosophers have
believed, the principles underlying the sciences were capable, when
disengaged from irrelevant detail, of giving us knowledge concerning
the universe as a whole, such knowledge would have the same claim on our
belief as scientific knowledge has; but our inquiry has not revealed any
such knowledge, and therefore, as regards the special doctrines of the
bolder metaphysicians, has had a mainly negative result. But as regards
what would be commonly accepted as knowledge, our result is in the main
positive: we have seldom found reason to reject such knowledge as the
result of our criticism, and we have seen no reason t