 This raises a difficult question. Kant contends that transcendental idealism as a general conception of space is to be contrasted with transcendental realism, a position he attributes both to Newton and to Leibniz. Yet from Leibniz’s point of view, the only elements of reality are substances and their properties. Relations, which are neither substances nor properties, are not among such elements, if we are speaking with metaphysical rigor. This is evident in a number of texts; for instance, in response to his Lockean interlocutor, Leibniz writes in the New Essays: “This division of the objects of our thoughts into substances, modes, and relations is pretty much to my liking. I believe that qualities are nothing but modifications of substances and the understanding adds relations [l’entendement y adjoute les relations].” He adds that relations are “the work of the understanding” (New Essays, 145). Since space for Leibniz is just the order of object relations, and relations are ideal, space too is ideal in some sense (New Essays, 145; cf. Cassirer 1902, 248). That is to say, there is a sense in which spatiality does not characterize reality (Adams 1994, 254–5). These views are to be found in the letters to Clarke and in the New Essays, both of which Kant read, and Kant himself explicitly mentions some of  them.[19]  The obvious obstacles to understanding Leibniz as a realist complicate the interpretation of transcendental idealism, since Kant clearly thinks it is an alternative to both Leibniz’s and Newton’s allegedly realist views. This problem is tackled in the next section.