 Leibniz’s theory of soul-body pre-established harmony set out in his New System of the Nature and the Communication of Substances of 1695 is not easy to reconcile with the interpretation of his monadology according to which what we call bodies are appearances in visual space founded upon spiritual substances lying underneath the spatio-temporal order. Yet the pre-established harmony was at least consistent with Leibniz’s claim that substances do not interact with one another and that what we call “causal interaction” does not involve a flow of power or force, but merely a regular sequence of changes in two observable things, in the case of mind and body, the experiences of perception and appetition and states of the sensory organs (G 4:76–7). Kant points to the tension between the theory of pre-established harmony and the monadology; “Why should one admit bodies, if it is possible that everything happens in the soul as a result of its own powers, which would run the same course even if entirely isolated?” (8:249).