It is perhaps unnecessary to make any connection between A. J. Ayer and Karl R. Popper other than to point out that they both had great influence on Western philosophy during the middle part of this century, an influence that has continued to this day. However, a common historical and intellectual connection is the Vienna Circle; this was a group that met in Vienna during the 1920s and 1930s and developed the philosophy of logical positivism, which was intent on setting philosophy on a sure footing so that the scope of its tasks was clear. Logical positivism, by way of a theory of meaning, involves the elimination of much of traditional philosophy, in particular metaphysics and also theology, as literally meaningless. What this amounted to was the view that the investigation of any substantial facts about the world was the province of science alone, not philosophy, which could be concerned only with conceptual elucidation and the linguistic task of precise definition. Both Ayer and Popper attended the meetings of the Vienna Circle, but whereas Ayer initially became a powerful advocate of its views, Popper, although deeply interested, like the Vienna Circle, in the philosophy and methodology of science, was critical of logical positivism. Popper aims to demarcate science from non-science so as to understand better the nature of scientific knowledge. Non-science includes pseudo-science: areas which are not scientific but claim to be so. It does not follow from this that what is non-science, including pseudo-science, is thereby literally meaningless, as logical positivism supposed, or even that it is untrue. Ayer has always had a great interest in the problem of meaning, which Popper regards as a largely fruitless field of philosophical investigation if regarded as an end in itself. What perhaps unites Ayer and Popper, although they are by no means alone in this, is their view that the heart of philosophy is epistemology, and in particular the nature of empirical knowledge.