If I remember Russells Short History of Western Philosophy correctly, Russell unequivocally maintains that Thales was the first philosopher of note. He said:
The world is made of water
Russell does not explain in any detail as to why he held this view, saying it was most likely he considered water as an 'originating' principle. In fact it is only in Aristotles Metaphysics that we have any evidence that Thales made this claim, and Aristotle then goes onto conjecture what he may mean by this.
That from which is everything that exists and from which it first becomes and into which it is rendered at last, its substance remaining under it, but transforming in qualities, that they say is the element and principle of things that are. …For it is necessary that there be some nature (φύσις), either one or more than one, from which become the other things of the object being saved... Thales the founder of this type of philosophy says that it is water
That is it's roughly equivalent to our idea of matter; Anaximenes, a later philosopher chose air rather than water (notably this is two of the four elements), and Anaximander, his student etherealised it into the apeiron.
Why would Russell choose water as an 'originary' principle rather than a 'matter' principle?
Particularly since then a theory of matter had already been worked out and was at hand. Was there in fact two competing explanations, and Russell chose the 'originary' one? Had he not read Aristotle, or was he working from bad translations (assuming he had little greek)?