.     FIRST LECTURE.
Introduction: Reversibility and Irreversibility.

Colleagues, ladies and gentlemen: The cordial invitation, which
the President of Columbia University extended to me to
deliver at this prominent center of American science some
lectures in the domain of theoretical physics, has inspired in
me a sense of the high honor and distinction thus conferred
upon me and, in no less degree, a consciousness of the
special obligations which, through its acceptance, would be
imposed upon me. If I am to count upon meeting in some
measure your just expectations, I can succeed only through
directing your attention to the branches of my science with
which I myself have been specially and deeply concerned, thus
exposing myself to the danger that my report in certain respects
shall thereby have somewhat too subjective a coloring.

From those points of view which appear to me the most
striking, it is my desire to depict for you in these lectures the
present status of the system of theoretical physics. I do not
say: the present status of theoretical physics; for to cover this
far broader subject, even approximately, the number of lecture
hours at my disposal would by no means suffice. Time limita¬
tions forbid the extensive consideration of the details of this great
field of learning; but it will be quite possible to develop for you, in
bold outline, a representation of the system as a whole, that is, to
give a sketch of the fundamental laws which rule in the physics
of today, of the most important hypotheses employed, and of
the great ideas which have recently forced themselves into the
subject. I will often gladly endeavor to go into details, but not
in the sense of a thorough treatment of the subject, and only with
the object of making the general laws more clear, through appro-

1