ly near alternative path. Mathematicians
would express this by saying, that the integral impetus is stationary
for an infinitesimal displacement. In this statement of the law of
motion I have neglected the existence of other forces. But that would
lead me too far afield.

The electromagnetic theory has to be modified to allow for the presence
of a gravitational field. Thus Einstein's investigations lead to the
first discovery of any relation between gravity and other physical
phenomena. In the form in which I have put this modification, we deduce
Einstein's fundamental principle, as to the motion of light along its
rays, as a first approximation which is absolutely true for infinitely
short waves. Einstein's principle, thus partially verified, stated in my
language is that a ray of light always follows a path such that the
integral impetus along it is zero. This involves that every element of
impetus along it is zero.

In conclusion, I must apologise. In the first place I have considerably
toned down the various exciting peculiarities of the original theory and
have reduced it to a greater conformity with the older physics. I do not
allow that physical phenomena are due to oddities of space. Also I have
added to the dullness of the lecture by my respect for the audience. You
would have enjoyed a more popular lecture with illustrations of
delightful paradoxes. But I know also that you are serious students who
are here because you really want to know how the new theories may affect
your scientific researches.

CHAPTER IX

THE ULTIMATE PHYSICAL CONCEPTS

The second chapter of this book lays down the first principle to be
guarded in framing our physical concept. We must avoid vicious
bifurcation. Nature is nothing else than the deliverance of
sense-awareness. We have no principles whatever to tell us what could
stimulate mind towards sense-awareness. Our sole task is to exhibit in
one system the characters and inter-relations of all that is observed.
Our attitude towards nature is purely 'behaviouristic,' so far as
concerns the formulation of physical concepts.

Our knowledge of nature is an experience of activity (or passage). The
things previously observed are active entities, the 'events.' They are
chunks in the life of nature. These events have to each other relations
which in our knowledge differentiate themselves into space-relations and
time-relations. But this differentiation between space and time, t