How did the testability criterion lead logical empiricists to reject speculative metaphysics?

Logical empiricists, including Rudolf Carnap and the Vienna Circle, utilized the testability criterion (or verifiability principle) to distinguish between meaningful factual assertions and meaningless speculation. They argued that for a non-analytic statement to be cognitively meaningful, it must be possible, in principle, to identify empirical observations that would count for or against its truth.12

Metaphysical and theological doctrines were rejected not because they were necessarily false, but because they failed this criterion. When challenged, metaphysicians often proved unable to specify what evidence could refute their claims, behaving as if their contentions were compatible with any possible state of affairs. Because these statements lacked observational implications and could not be confirmed or disconfirmed by experience, logical empiricists concluded they made no significant assertions at all. Consequently, such systems were viewed as essentially idle—lacking factual meaning and serving only to express emotions rather than provide genuine understanding of the world.341