While the members of the school of Logical Empiricism may differ in various details, nearly all of them are opposed to metaphysics on the ground that a scientific metaphysics is the only possible one. All philosophy is to become scientific. This assumption is based on their epistemological criterion of verifiability which appears to be a basic doctrine of this school. The implications of this doctrine have not been worked out in detail by many, but one of the most explicit accounts has been given by A. J. Ayer in his book on Language, Truth and Logic, which can be taken as a representative work in its attack on metaphysics.