See link for main page on logical positivism
The emergence of scientific knowledge in conflict with, and as a rival to, religious knowledge, led to a study of the “demarcation problem” – how to differentiate (and prove the superiority of) scientific knowledge from other types of knowledge. This program reached a highly successful culmination with the emergence of the philosophy of logical positivism in the early twentieth century. Here “successful” means that the philosophy was overwhelmingly accepted by scholars for a large part of the twentieth century, not that it was correct. Indeed, subsequent investigations revealed so many difficulties that even its main proponents were forced admit[1] that it was nearly “all wrong”. For example, a modern empiricist Van Fraassen (1980, p. 2) writes: “Logical positivism, … even if one is quite charitable … had a rather spectacular crash.” Suppe (1977) provides the epitaph, a detailed and comprehensive discussion of reasons why empiricism was eventually abandoned by philosophers.
According to the positivist philosophy, scientific statements were based on observations and logical deductions from them. Statements which could not be verified or disconfirmed by observations were meaningless. In particular, values, ethics and moral judgments were not scientific, and in effect meaningless, except as an expression of an emotional attachment. This effectively relegated a huge portion of existing knowledge, which included religious knowledge, to the dustbin. Julie Reuben (1996) writes that:
In the late nineteenth century, intellectuals assumed that truth had spiritual, moral and cognitive dimensions. By 1930, however, intellectuals had abandoned this broad conception of truth. They embraced, instead, a view of knowledge that drew a sharp distinction between “facts” and “values”. They associated cognitive truth with empirically verified knowledge and maintained that by this standard, moral values could not be validated as “true.” In the nomenclature of the twentieth century, only “science” constituted true knowledge. Moral or spiritual values could be “true” in an emotional or nonliteral sense, but not in terms of cognitively verifiable knowledge. The term “truth” no longer comfortably encompassed factual knowledge and moral values.
In this section, we briefly discuss three powerful and widely believed positivist arguments for keeping values out of scientific discourse. Variants of all three are contained in the following quote from Ayer (1936) :
We can now see why it is impossible to find a criterion for determining the validity of ethical judgements. It is not because they have an ‘absolute’ validity which is mysteriously independent of ordinary sense-experience, but because they have no objective validity whatsoever . . . They are pure expressions of feeling and as such do not come under the category of truth and falsehood. They are unverifiable for the same reason as a cry of pain or a word of command is unverifiable[as a statement] – because they do not express genuine propositions.
[1] Ayer himself in later life is supposed to have remarked about Language, Truth, and Logic that it was “all wrong” – http://www.newworldencyclopedia.org/entry/A.J._Ayer Accessed 25 Sep 2009.