In philosophy of science and epistemology, the demarcation problem is the question of how to distinguish between science and non-science. It examines the boundaries between science, pseudoscience and other products of human activity, like art and literature and beliefs. The debate continues after over two millennia of dialogue among philosophers of science and scientists in various fields. The debate has consequences for what can be called "scientific" in fields such as education and public policy. The ancient world An early attempt at demarcation can be seen in the efforts of Greek natural philosophers and medical practitioners to distinguish their methods and their accounts of nature from the mythological or mystical accounts of their predecessors and contemporaries. G. E. R. Lloyd noted that there was a sense in which the groups engaged in various forms of inquiry into nature set out to "legitimate their own positions", laying "claim to a new kind of wisdom ... that purported to yield superior enlightenment, even superior practical effectiveness". Medical writers in the Hippocratic tradition maintained that their discussions were based on necessary demonstrations, a theme developed by Aristotle in his Posterior Analytics. One element of this polemic for science was an insistence on a clear and unequivocal presentation of arguments, rejecting the imagery, analogy, and myth of the old wisdom. Some of their claimed naturalistic explanations of phenomena have been found to be quite fanciful, with little reliance on actual observations. Cicero's De Divinatione implicitly used five criteria of scientific demarcation that are also used by modern philosophers of science. Logical positivism Logical positivism, formulated in the 1920s, held that only statements about matters of fact or logical relations between concepts are meaningful. All other statements lack sense and are labelled "metaphysics" (see the verifiability theory of meaning also known as verificationism). According to A. J. Ayer, metaphysicians make statements which claim to have "knowledge of a reality which [transcends] the phenomenal world". Ayer, a member of the Vienna Circle and a noted English logical-positivist, argued that making any statements about the world beyond one's immediate sense-perception is impossible. This is because even metaphysicians' first premises will necessarily begin with observations made through sense-perception. Ayer implied that the line of demarcation is characterized as the place at which statements become "factually significant". To be "factually significant", a statement must be verifiable. In order to be verifiable, the statement must be verifiable in the observable