 On the other hand, all of Carnap’s repeated attempts, over several decades, to give a precise delineation—to explicate the distinction—between scientific (or meaningful or communicatively adequate) statements and metaphysical (non-scientific, non-meaningful) ones failed. He therefore became more liberal and less exacting about what was to be regarded as metaphysical. He was willing to allow that many metaphysical theories of past philosophers (he cites Aristotle, Leibniz, Kant, Peirce, and Whitehead, among others) could be regarded as useful steps toward the construction of “the most general frameworks containing categorial concepts which are fundamental for the representation of all knowledge”, and moreover that he himself regarded this task as among “the most important problems of philosophy” (Carnap 1963b: 862). See the supplement  Tolerance, Metaphysics, and Meta-Ontology (Section 2 and 3)  for further detail.