he long tradition of
scientific philosophy. For example in the _Timaeus_ there is a
presupposition, somewhat vaguely expressed, of a distinction between the
general becoming of nature and the measurable time of nature. In a later
lecture I have to distinguish between what I call the passage of nature
and particular time-systems which exhibit certain characteristics of
that passage. I will not go so far as to claim Plato in direct support
of this doctrine, but I do think that the sections of the _Timaeus_
which deal with time become clearer if my distinction is admitted.

This is however a digression. I am now concerned with the origin of the
scientific doctrine of matter in Greek thought. In the _Timaeus_ Plato
asserts that nature is made of fire and earth with air and water as
intermediate between them, so that 'as fire is to air so is air to
water, and as air is to water so is water to earth.' He also suggests a
molecular hypothesis for these four elements. In this hypothesis
everything depends on the shape of the atoms; for earth it is cubical
and for fire it is pyramidal. To-day physicists are again discussing
the structure of the atom, and its shape is no slight factor in that
structure. Plato's guesses read much more fantastically than does
Aristotle's systematic analysis; but in some ways they are more
valuable. The main outline of his ideas is comparable with that of
modern science. It embodies concepts which any theory of natural
philosophy must retain and in some sense must explain. Aristotle asked
the fundamental question, What do we mean by 'substance'? Here the
reaction between his philosophy and his logic worked very unfortunately.
In his logic, the fundamental type of affirmative proposition is the
attribution of a predicate to a subject. Accordingly, amid the many
current uses of the term 'substance' which he analyses, he emphasises
its meaning as 'the ultimate substratum which is no longer predicated of
anything else.'

The unquestioned acceptance of the Aristotelian logic has led to an
ingrained tendency to postulate a substratum for whatever is disclosed
in sense-awareness, namely, to look below what we are aware of for the
substance in the sense of the 'concrete thing.' This is the origin of
the modern scientific concept of matter and of ether, namely they are
the outcome of this insistent habit of postulation.

Accordingly ether has been invented by modern science as the substratum
of the events