"The question of the relationship between words and physical, sensory reality was taken up by the Vienna circle of logical positivists, whose leading members included Moritz Schlick, a successor of Mach at the University of Vienna and teacher of Hayek; Otto Neurath, the group’s practical organizer; and Rudolf Carnap, another leading Viennese philosopher. Hayek was not a member of the Vienna circle, though he was aware of discussions there as a result of his friendship with Felix Kaufmann, a member of the circle, Hayek’s Geistkreis (spirit circle), and Mises’s “private seminar.”While it is always difficult to summarize the philosophy of a school that included diverse members over decades, prominent logical positivist themes included the essentiality of verification to knowledge; the exclusive meaningfulness of mathematics, logic, and science in knowledge; and the rejection, as knowledge, of ethics, metaphysics, and religion. Logical positivists were concerned with the philosophy of science and foundations of knowledge. They strove to answer the question: What makes something true? They drew heavily on the work of earlier British empiricists, such as Hume, and more recent philosophers of mathematics, logic, and semantics such as Wittgenstein and Bertrand Russell. They were also inspired by Einstein.The key ideas of this group of philosophers, scientists, and mathematicians included verificationism and empiricism—that is, knowledge must be capable of being proven to the senses in order to be scientific, in order to be knowledge."