ferentiation. Some objects we know by sight only, and other objects
we know by sound only, and other objects we observe neither by light nor
by sound but by touch or smell or otherwise. The velocity of light
varies according to its medium and so does that of sound. Light moves in
curved paths under certain conditions and so does sound. Both light and
sound are waves of disturbance in the physical characters of events; and
(as has been stated above, p. 188) the actual course of the light is of
no more importance for perception than is the actual course of the
sound. To base the whole philosophy of nature upon light is a baseless
assumption. The Michelson-Morley and analogous experiments show that
within the limits of our inexactitude of observation the velocity of
light is an approximation to the critical velocity 'c' which expresses
the relation between our space and time units. It is provable that the
assumption as to light by which these experiments and the influence of
the gravitational field on the light-rays are explained is deducible _as
an approximation_ from the equations of the electromagnetic field. This
completely disposes of any necessity for differentiating light from
other physical phenomena as possessing any peculiar fundamental
character.

It is to be observed that the measurement of extended nature by means of
extended objects is meaningless apart from some observed fact of
simultaneity inherent in nature and not merely a play of thought.
Otherwise there is no meaning to the concept of one presentation of your
extended measuring rod AB. Why not AB' where B' is the end B
five minutes later? Measurement presupposes for its possibility nature
as a simultaneity, and an observed object present then and present now.
In other words, measurement of extended nature requires some inherent
character in nature affording a rule of presentation of events.
Furthermore congruence cannot be defined by the permanence of the
measuring rod. The permanence is itself meaningless apart from some
immediate judgment of self-congruence. Otherwise how is an elastic
string differentiated from a rigid measuring rod? Each remains the same
self-identical object. Why is one a possible measuring rod and the other
not so? The meaning of congruence lies beyond the self-identity of the
object. In other words measurement presupposes the measurable, and the
theory of the measurable is the theory of congruence.

Furthermore the admission of