In Russell's Problems of Philosophy (PP), acquaintance is the basis of thought and also the basis of empirical knowledge. Thought is based on acquaintance, in that a thinker has to be acquainted with the basic constituents of his thoughts. Empirical knowledge is based on acquaintance, in that acquaintance is involved in perception, and perception is the ultimate source of all empirical knowledge.

It might seem that there are two separate questions about universals, the question of what they are and the question why we should believe that there are such things, and that the former question should be taken first; it might seem that until you know what they are it cannot be sensible to ask whether one should believe in them. How, for example, could one know whether it was sensible or even possible to believe in Father Christmas until one knew who or what he was supposed to be? But appearances could be deceptive. In the case of universals the position is different. What happened was that philosophers found themselves faced with certain problems of which they were inclined to say: this problem is insoluble unless there are some entities which have certain characteristics, the characteristics which would enable the problem to be solved. The things which, if they existed, would solve their problems they called forms or universals. So universals are things which have whatever properties they need to have to solve certain problems. This being so, it is clearly sensible to approach the theory of universals from the problems which led to philosophers postulating their existence.

Alfred Jules Ayer (1910– ) was born in London and educated at Eton and Christ Church, Oxford. He attended sessions of the logical positivist ‘Vienna Circle’ in 1932, and taught at Oxford from 1933 until joining the Army in 1940. His Language, Truth and Logic was published in 1936, and The Foundations of Empirical Knowledge in 1940. After war service he returned to Oxford in 1945, and became Grote Professor of the Philosophy of Mind and Logic at University College, London, the following year. The Problem of Knowledge was published in 1956. In 1959 he returned to Oxord as Wykeham Professor of Logic, a post he held until his retirement in 1977. He had been made a Fellow of the British Academy in 1952, and was knighted in 1970. Among his publications after he returned to Oxford are The Concept of a Person (1963), Philosophical Essays (1965), The Origins of Pragmatism (1968), Metaphysics and Common Sense (1969), Russell and Moore: the Analytical Heritage (1971), Probability and Evidence (1972), The Central Questions of Philosophy (1973), and Philosophy in the Twentieth Century (1982).

When, in 1979, A. J. Ayer was asked for an evaluation of his youthful Language, Truth and Logic (LTL), he replied: ‘I suppose the most important of the defects was that nearly all of it was false’. Like many of the claims in the book itself, this verdict is open to question. What was wrong with LTL was not so much that what it said was false, but that it presented philosophical issues in an excessively simple and aggressive way. Yet it was just this quality that put the book and its author on the philosophical map, ensuring for them an important place in the history of twentieth-century philosophy. LTL presented a challenge to traditional ways of doing philosophy, the reverberations of which are still evident today.

Jean-Paul Sartre (1905–1980), nephew of the Alsatian theologian, Albert Schweitzer, was born in Paris, passed his agrégation at the Ecole Normale Superieure in 1929, and was a lycée teacher between 1931 and 1945. He was called up to the French Army in 1939, captured by the Germans in 1940 and released after the armistice. In 1938 he published a novel, La Nausée, translated by Robert Baldick as Nausea (Harmondsworth: Penguin, 1965), and in 1940, L'Imaginaire: Psychologie phénoménologique de l'imagination, translated by Bernard Frechtman as The Psychology of Imagination (London: Methuen, 1972). His major philosophical work, L'Etre et le Neant, was published in 1943, and translated by Hazel E. Barnes as Being and Nothingness (London: Methuen, 1957). As a novelist he is best known for a trilogy, Chemins de la Liberté (Roads to Freedom), comprising L'Age de raison (1945) translated by E. Sutton as The Age of Reason (Harmondsworth: Penguin, 1961), Le Sursis (1945), translated by E. Sutton as The Reprieve (Harmondsworth: Penguin, 1963) and La Mort dans l'āme (1949), translated by G. Hopkins as Iron in the Soul (Harmondsworth: Penguin, 1965). His main work of literary criticism is Qu'est-ce que la littérature? (1947), translated by B. Frechtman as What is Literature? (London: Methuen, 1950). Plays includeLes Mouches (1943) and Huis Clos (1944), both translated by S. Gilbert and published in one volume, as The Flies and In Camera (London: Hamish Hamilton, 1965).

Sartre presented ‘Existentialism and Humanism’ to a popular audience in Paris late in 1945. As he implies in the discussion which is appended to the text of the lecture (pp. 57–58), he was here simplifying his views so as to make them intelligible to a wide audience. In this he succeeded only too well; the lecture has become exceedingly well known and has been regarded as a definitive presentation not only of Sartre's philosophy at the time, but also of ‘existentialism’. One thing I hope to show in this essay is that this is not a sensible view to take; Sartre's text requires a good deal of interpretation and qualification in the light of his other writings of the period, and what emerges is a position which is uniquely his own. One way in which this can be seen is by considering Heidegger's ‘Letter on Humanism’ of 1947 which is a response to Sartre's lecture and is, indeed, Heidegger's only direct response to Sartre's work. In the lecture Sartre had associated Heidegger with himself as an ‘existential atheist’ (p. 26), but in his letter Heidegger emphatically dissociates himself both from atheism and from existentialism as characterized by Sartre, and goes on to criticize the position advanced by Sartre in the lecture. Yet despite the popular exaggeration of the significance of Sartre's lecture, it is certainly worth studying; for not only is it short and accessible, though in some respects misleading, it is also one of Sartre's few indications of the positive ethical theory which so many of his writings require but do not supply.