The Critique of Pure Reason | Page 5

logical science. We do not enlarge but disfigure
the sciences when we lose sight of their respective limits and allow
them to run into one another. Now logic is enclosed within limits which
admit of perfectly clear definition; it is a science which has for its
object nothing but the exposition and proof of the formal laws of all
thought, whether it be a priori or empirical, whatever be its origin or its
object, and whatever the difficulties--natural or accidental-- which it
encounters in the human mind.
The early success of logic must be attributed exclusively to the
narrowness of its field, in which abstraction may, or rather must, be
made of all the objects of cognition with their characteristic distinctions,
and in which the understanding has only to deal with itself and with its
own forms. It is, obviously, a much more difficult task for reason to
strike into the sure path of science, where it has to deal not simply with
itself, but with objects external to itself. Hence, logic is properly only a
propaedeutic--forms, as it were, the vestibule of the sciences; and while
it is necessary to enable us to form a correct judgement with regard to
the various branches of knowledge, still the acquisition of real,
substantive knowledge is to be sought only in the sciences properly so
called, that is, in the objective sciences.
Now these sciences, if they can be termed rational at all, must contain

elements of a priori cognition, and this cognition may stand in a
twofold relation to its object. Either it may have to determine the
conception of the object--which must be supplied extraneously, or it
may have to establish its reality. The former is theoretical, the latter
practical, rational cognition. In both, the pure or a priori element must
be treated first, and must be carefully distinguished from that which is
supplied from other sources. Any other method can only lead to
irremediable confusion.
Mathematics and physics are the two theoretical sciences which have to
determine their objects a priori. The former is purely a priori, the latter
is partially so, but is also dependent on other sources of cognition.
In the earliest times of which history affords us any record,
mathematics had already entered on the sure course of science, among
that wonderful nation, the Greeks. Still it is not to be supposed that it
was as easy for this science to strike into, or rather to construct for itself,
that royal road, as it was for logic, in which reason has only to deal
with itself. On the contrary, I believe that it must have remained
long--chiefly among the Egyptians--in the stage of blind groping after
its true aims and destination, and that it was revolutionized by the
happy idea of one man, who struck out and determined for all time the
path which this science must follow, and which admits of an indefinite
advancement. The history of this intellectual revolution--much more
important in its results than the discovery of the passage round the
celebrated Cape of Good Hope--and of its author, has not been
preserved. But Diogenes Laertius, in naming the supposed discoverer
of some of the simplest elements of geometrical
demonstration--elements which, according to the ordinary opinion, do
not even require to be proved--makes it apparent that the change
introduced by the first indication of this new path, must have seemed of
the utmost importance to the mathematicians of that age, and it has thus
been secured against the chance of oblivion. A new light must have
flashed on the mind of the first man (Thales, or whatever may have
been his name) who demonstrated the properties of the isosceles
triangle. For he found that it was not sufficient to meditate on the figure,
as it lay before his eyes, or the conception of it, as it existed in his mind,

and thus endeavour to get at the knowledge of its properties, but that it
was necessary to produce these properties, as it were, by a positive a
priori construction; and that, in order to arrive with certainty at a priori
cognition, he must not attribute to the object any other properties than
those which necessarily followed from that which he had himself, in
accordance with his conception, placed in the object.
A much longer period elapsed before physics entered on the highway of
science. For it is only about a century and a half since the wise Bacon
gave a new direction to physical studies, or rather--as others were
already on the right track--imparted fresh vigour to the pursuit of this
new direction. Here, too, as in the case of mathematics, we find
evidence of a rapid intellectual revolution. In the remarks which follow
I shall confine myself to the empirical side of natural science.