In THE PROBLEMS OF PHILOSOPHY is one of Bertrand Russell's attempts to create a brief and accessible guide to the contemporary problems of late 19th and early 20th century philosophy. Focusing on problems he believes will provoke positive and constructive discussion, Russell concentrates on knowledge rather than metaphysics: If it is uncertain that external objects exist, how can we then have knowledge of them but by probability. There is no reason to doubt the existence of external objects simply because of sense data.