Friedrich Nietzsche argued that Kant commits an agnostic tautology and does not offer a satisfactory answer as to the source of a philosophical right to such-or-other metaphysical claims; he ridicules his pride in tackling "the most difficult thing that could ever be undertaken on behalf of metaphysics." The famous "thing-in-itself" was called a product of philosophical habit, which seeks to introduce a grammatical subject: because wherever there is cognition, there must be a thing that is cognized and allegedly it must be added to ontology as a being (whereas, to Nietzsche, only the world as ever changing appearances can be assumed). Yet he attacks the idealism of Schopenhauer and Descartes with an argument similar to Kant's critique of the latter (see above).