Problems about perception have long exercised philosophers. They offer for that reason alone a good example of the change in approach which distinguishes traditional from revolutionary philosophy. Philosophers have traditionally asked about the relation between perception and knowledge. Ayer’s first point is that the argument from illusion is inconclusive, if its claim that the people are never directly aware of external objects is regarded as a matter of fact. In Ayer’s second argument he goes on to consider, primarily for illustrative purposes, the grounds for accepting the original premise in the argument from illusion, that the people sometimes have perceptual illusions. Ayer’s description of the background conventions as linguistic consequently has some value. Perhaps the description of the background conventions as linguistic has some contribution to make to an understanding of their function and status.