riation of
the velocity of light in a moving medium is explained by the formulae of
the third case, and requires another arbitrary assumption if we use the
orthodox formulae.

It appears therefore that on the mere basis of physical explanation
there are advantages in the formulae of the third case as compared with
the orthodox formulae. But the way is blocked by the ingrained belief
that these latter formulae possess a character of necessity. It is
therefore an urgent requisite for physical science and for philosophy to
examine critically the grounds for this supposed necessity. The only
satisfactory method of scrutiny is to recur to the first principles of
our knowledge of nature. This is exactly what I am endeavouring to do in
these lectures. I ask what it is that we are aware of in our
sense-perception of nature. I then proceed to examine those factors in
nature which lead us to conceive nature as occupying space and
persisting through time. This procedure has led us to an investigation
of the characters of space and time. It results from these
investigations that the formulae of the third case and the orthodox
formulae are on a level as possible formulae resulting from the basic
character of our knowledge of nature. The orthodox formulae have thus
lost any advantage as to necessity which they enjoyed over the serial
group. The way is thus open to adopt whichever of the two groups best
accords with observation.

I take this opportunity of pausing for a moment from the course of my
argument, and of reflecting on the general character which my doctrine
ascribes to some familiar concepts of science. I have no doubt that some
of you have felt that in certain aspects this character is very
paradoxical.

This vein of paradox is partly due to the fact that educated language
has been made to conform to the prevalent orthodox theory. We are thus,
in expounding an alternative doctrine, driven to the use of either
strange terms or of familiar words with unusual meanings. This victory
of the orthodox theory over language is very natural. Events are named
after the prominent objects situated in them, and thus both in language
and in thought the event sinks behind the object, and becomes the mere
play of its relations. The theory of space is then converted into a
theory of the relations of objects instead of a theory of the relations
of events. But objects have not the passage of events. Accordingly space
as a relation between