in the properties of light which have led to the undulatory theory.
Let us devote a little while to the consideration of these two
subjects.

Outside of physics we know nothing of action at a distance. When
we try to connect cause and effect in the experiences which natural
objects afford us, it seems at first as if there were no other mutual
actions than those of immediate contact, e.g. the communication of
motion by impact, push and pull, heating or inducing combustion by
means of a flame, etc. It is true that even in everyday experience
weight, which is in a sense action at a distance, plays a very
important part. But since in daily experience the weight of bodies
meets us as something constant, something not linked to any cause
which is variable in time or place, we do not in everyday life
speculate as to the cause of gravity, and therefore do not become
conscious of its character as action at a distance. It was Newton's
theory of gravitation that first assigned a cause for gravity by
interpreting it as action at a distance, proceeding from masses.
Newton's theory is probably the greatest stride ever made in
the effort towards the causal nexus of natural phenomena. And yet
this theory evoked a lively sense of discomfort among Newton's
contemporaries, because it seemed to be in conflict with the
principle springing from the rest of experience, that there can be
reciprocal action only through contact, and not through immediate
action at a distance. It is only with reluctance that man's desire
for knowledge endures a dualism of this kind. How was unity to
be preserved in his comprehension of the forces of nature? Either
by trying to look upon contact forces as being themselves distant
forces which admittedly are observable only at a very small
distance--and this was the road which Newton's followers, who were