The reputation and influence of Mr. Bertrand Russell among contemporary philosophers writing in English, and the persistency and originality of his efforts to find a way of escape from at least one of the forms of dualism, make it seem desirable, for the purpose of these lectures, to devote to his philosophy, as to Mr. Whitehead's, an especially careful examination. The initial and decisive measure is the adoption of a new definition of a "physical object" or a "piece of matter". The theory of the constitution of matter set forth in Mr. Russell's earlier doctrine is not merely accidentally but necessarily different from that of the physicist, because it expresses precisely the opposite philosophical motive. It must have been evident to every observant reader that Mr. Russell, in his expositions of this theory, was quite unable to dispense with the concepts and modes of reasoning which he professed to abjure.