How did logical positivists distinguish scientific statements from metaphysical statements?

Logical positivists, including A.J. Ayer, distinguished scientific statements from metaphysical ones using the verifiability principle. This doctrine asserts that a statement is cognitively meaningful only if it is either tautological (true by definition) or empirically verifiable.12

To determine if a statement is meaningful, positivists asked what observable conditions would count for or against its truth. Scientific statements are considered meaningful because they relate to empirical facts and can be supported or rendered probable by observation. In contrast, metaphysical statements—such as those claiming to reveal absolute truths—were dismissed as meaningless because they are often compatible with any possible state of affairs, meaning no empirical evidence could ever refute them.12

While the Vienna Circle initially sought a strict criterion for verification, they faced significant challenges in formulating one that was neither too restrictive (excluding valid science) nor too lax (admitting metaphysical claims). Consequently, the movement evolved, with thinkers like Ayer attempting to refine the definition of a "genuine factual proposition" to better align with the requirements of scientific inquiry.23