It was thought that there was need for some comprehensive work
in the English language on the philosophy of the exact sciences
which would do full justice to the newer developments in mathe-
matical and physical speculation while showing at the same time
the historical connections of these tendencies. It seemed that the
two works of Professor Ernst Cassirer herewith presented fulfilled
these requirements best of all. The reader will find here a construc-
tive and systematic survey of the whole field of the principles of
the exact sciences from the standpoint of a logical idealism, which
is historically derived from Kant, but which lacks the fatal rigidity
of the latter's system. As Professor Cassirer develops his logical or
critical idealism it becomes a doctrine of creative intelligence. His
doctrine is neither idealism, pragmatism nor realism as these terms
are understood in our English-speaking philosophy; it is rather a
positivistic and non-static rationalism, which seeks to preserve
the spirit which unites Plato, Descartes, Leibniz and Kant and to
show how this spirit reaches its fulfillment in the modern develop-
ment of mathematical and physical theory.

The first part of the present book, Substanzbegriff und Funktions-
begriff was published in 1910, while the second part, which we have
called the Supplement, Zur Einstein'schen Relativitätstheorie, ap-
peared in 1921. The intervening period was, of course, one of
immense importance for the philosophy of physics, since it marked
the development of the new and revolutionary theory of relativity.
In accordance with the fundamental maxim of his critical method
Professor Cassirer based his analysis in 1910 on the historical state
of science, which was still dominated by the Newtonian conceptions
of space and time. On the ground of the same maxim, he has since
taken account of the new theory of relativity and has, with good
logical justification, seen in the latter the relative completion and
realization of the historical tendency which he had described in his
earlier works. Professor Cassirer's philosophy may be regarded
as a fundamental epistemological "theory of relativity" which sets
forth a general philosophical standpoint from which Einstein's theory
is seen to be only the latest and most radical fulfillment of the motives

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