This is a representative collection of the work of A.J. Ayer, one of the most influential contemporary philosophers. It includes his Whidden lectures on freedom and morality, which were presented at McMaster University in 1983, a previously unpublished essay on J.L. Mackie's Theory of Causal Priority, and seven other essays which cover such topics as: references and identity, the causal theory of perception, the prisoner's paradox, self-evidence and certainty, and the history of the Vienna Circle.