The whole enterprise of metaphysics came under heavy attack in the 1920's and 30's from a group of philosophers originally based in Vienna who came to be known as Logical Positivists. Logical Positivism had its roots in the skepticism of David Hume. Hume rejected the claims to metaphysical knowledge. For example, Hume rejected belief in the existence of God or the immortality of the soul since according to Hume the ideas on which such claims regarding these subjects are based cannot be traced back to the simple sense impressions.
The Vienna Circle, including individuals such as Morris Schlick, A. J. Ayer, and Ludwig Wittgenstein, determined that all religious and ethical statements are nonsensical since such statements cannot be empirically verified. This criterion was given the designation The Principle of Verification. Prominent among the Logical Positivists was Rudolph Carnap. Carnap's position is that the traditional claims of the metaphysicians are, in the strict sense, meaningless and that they are "pseudo-statements" in that fail to assert anything at all since these statements cannot be verified.
The Logical Positivists in general and Carnap in particular essentially attempted to create a way for metaphysical speculation to die. However, such was not to be the case. It appears that Carnap recognized that absolute verification was impossible and that a mediating position must be developed regarding truth. Carnap determined it important that in absence of the possibility of absolute certainty regarding a proposition or statement, one is forced to explore the realm of possibility. Thus was born Carnap's continual search for a theory of and a method for the calculation of probability.
As a result Carnap determined that one must distinguish between two senses of probability. On the one hand there was" probability (1) which corresponds to credibility, and probability (2) which corresponds to the frequency or empirical conception of probability." (Cambridge Dictionary of Philosophy, "Carnap", 118)
Carnap, like his predecessors in the Vienna Circle, contended that many of the individual words used by metaphysicians are nonsensical. This is true since such words fail to meet the criterion of verification. For Carnap the only meaningful statements are either those which are true by virtue of their form, or those which are empirically or scientifically verifiable. Statements, which fail to meet these two criteria, are nonsensical or meaningless.
Carnap, however, preferred the term "problem of confirmation" rather than "problem of verification", arguing that a sentence may not be testable but may be confirmable. According to Carnap, a sentence is confirmable when we may know the method for confirmation but are unable to utilize the method for actual testing. (50 Major Philosophers, 156) For Carnap the only meaningful statements are those, which meet the following criteria:
1. There are statements, which are true solely by virtue of their form or are tautologically true. These statements correspond to Kant's analytic statements and these statements say nothing about reality.
2. There are the negations of such statements or contradictions. These types of statements are contradictory and are false by virtue of their very form or structure.
3. The truth or falsity of all other statements lie in the "protocol sentences" and these are true or false empirical statements and they belong to the domain of empirical science.
Carnap concludes, "Any statement which does not fall within these categories becomes automatically meaningless." (Carnap, "The Elimination of Metaphysics" in Western Philosophy: An Anthology, ed. Jon Cottingham, 125).
Carnap concluded that traditional metaphysical statements are worse than fairy tales in that at least fairy tales are at least capable of verification or falsifiability. According to Carnap metaphysical assertions fail to make any intelligible statements whatsoever. Metaphysics rejects analytic statements and does not utilize empirical methodology. Therefore metaphysics is forced to employ pseudo language. (Carnap, "The Elimination of Metaphysics" in Western Philosophy: An Anthology, ed. John Cottingham, 125) Carnap essentially reduced the complete task of Philosophy to nothing more than "the business of linguistic analysis." (Philosophy: 100 Essential Thinkers, 173)
Despite the vigorous and in some ways salutary challenge it offered, the program of the positivists for the elimination of metaphysics had ground to a halt by the 1960's. There turned out to be serious problems in formulating the verification principle in a way, which was stringent enough to exclude traditional metaphysics yet liberal enough to accommodate the complex theoretical statements of natural science.
However it should be pointed out that Logical Positivism seems to have died at the hands of its own criteria. In short, how does one verify the verification principle? In addition, the theoretical dimensions of the scientific endeavor also failed to pass the call of Logical Positivism for determining truth only through that which is empirically verifiable.
