Bertrand Russell (1872-1970) always tried to reconcile the analytical approach of science and the philosophical need for new provisional hypotheses about the universe: ‘science is what you more or less know and philosophy is what you do not know’. Only science is capable of giving an objective account of the physical world but, at the same time, philosophy, alone, is free to question or justify the nature, role and limitations of scientific concepts, as A.J Ayer’s ‘criterion of verifiability’ amply demonstrates. Russell may have abandoned his early project of an ideal, ‘logically perfect language’, but he never lost sight of his original quest for a better understanding of the true nature of physical and mental reality.

At the age of 87, the Nobel Prize for Literature judged all human knowledge ‘uncertain, inexact, and partial’ but was still prepared to challenge his long-standing views by introducing a new distinction between ‘experience’ and knowing’. If, he argued, ‘an experience is one thing and knowing it is another, the supposition that we always know an experience when it is happening involves an infinite multiplication of every event … with the consequence that we do not know our present experience’. In this sense, ‘knowing’ is divested of its traditional comprehensive claim as it simply comes down to the fact of being conscious of, or ‘noticing’, our present sensible field and showing an ‘appropriate’ reaction of our sense-organs. This constant paring down of concepts is the trademark of Russell’s scientific method of philosophical enquiry.