not guarantee an objective existence. Hume David Hume also criticized the distinction, although for reasons altogether fairly similar to those of Berkeley and Leibniz. In Book 1, Part 4 of A Treatise of Human Nature, he argues that we have no impressions of primary qualities at all, but rather only various impressions that we tend to group together into some particular mind-independent quality. Thus, according to Hume, primary qualities collapse into secondary qualities, making the distinction far less helpful than it first might have seemed. Kant Immanuel Kant, in his Prolegomena to Any Future Metaphysics That Will Be Able to Present Itself as a Science, claimed that primary, as well as secondary, qualities are subjective. They are both mere appearances that are located in the mind of a knowing observer. In § 13, Remark II, he wrote: "Long before Locke's time, but assuredly since him, it has been generally assumed and granted without detriment to the actual existence of external things, that many of their predicates may be said to belong not to the things in themselves, but to their appearances, and to have no proper existence outside our representation. Heat, color, and taste, for instance, are of this kind. Now, if I go farther, and for weighty reasons rank as mere appearances the remaining qualities of bodies also, which are called primary, such as extension, place, and in general space, with all that which belongs to it (impenetrability or materiality, space, etc.)—no one in the least can adduce the reason of its being inadmissible." This follows directly from Kant's transcendental idealism, according to which space and time are mere forms of intuition, which means that any quality that can be attributed to the spatiotemporal objects of experience must be a quality of how things appear to us rather than of how things are in themselves. Thus, while Kant did not deny the existence of objects beyond all possible experience, he did deny the applicability of primary quality terms to things in themselves. See also Critical realism Empiricism Logical positivism Phenomenology Qualia Unobservables References Concepts in epistemology Concepts in metaphysics Quality Conceptual distinctions