Professor Ayer's new book1 is of very great interest. All the discussion is skilful, much of it is ingenious; the arguments ramify, but never get out of control; the prose is pleasant, the presentation polished and civilized. Once before, Professor Ayer investigated the foundations of empirical knowledge; and the central topics of his new book are, to a large extent, the same as those of his earlier one. These are topics which have been much discussed since 1940. Ayer takes account of many of these discussions, and his views, in consequence, have changed. They have not changed quite so radically as one might have expected. Ayer thinks that the old-style sceptic ought still to be given a run on the old course against the old competitors. If I am not mistaken, it is important to keep this guiding thought in mind in trying to understand his book. For it is easy to glide comfortably over the smooth surface of this prose without noticing transitions and modulations which, in a less expertly composed treatise, might have given one a jolt.