A number of individuals have suggested that much of metaphysics should be rejected.
In the 18th century, David Hume took an extreme position, arguing that all genuine knowledge involves either mathematics or matters of fact and that metaphysics, which goes beyond these, is worthless.
He concludes his Enquiry Concerning Human Understanding with the statement:
If we take in our hand any volume; of divinity or school metaphysics, for instance; let us ask, Does it contain any abstract reasoning concerning quantity or number? No. Does it contain any experimental reasoning concerning matter of fact and existence? No. Commit it then to the flames: for it can contain nothing but sophistry and illusion.
They argued that metaphysical statements are neither true nor false but meaningless since, according to their verifiability theory of meaning, a statement is meaningful only if there can be empirical evidence for or against it.
Carnap took a similar line with the controversy over the reality of the external world.
33 years after Hume’s Enquiry appeared, Immanuel Kant published his Critique of Pure Reason.
Though he followed Hume in rejecting much of previous metaphysics, he argued that there was still room for some synthetic a priori knowledge, concerned with matters of fact yet obtainable independent of experience.
These included fundamental structures of space, time, and causality.
He also argued for the freedom of the will and the existence of “things in themselves”, the ultimate (but unknowable) objects of experience.