 Thought is another attribute through which substance is conceived (IIp1). Since substance is unique, and the attributes simply various essences under which it is conceived, the series of finite modal causes in each attribute must operate strictly in parallel with one another (IIp7). For every modification of Thought there is a modification of Extension it mirrors, and vice versa; and the causal order of the one is perfectly matched with that of the other. The mind, a finite mode of Thought, is, under this parallelism, simply the idea of the body to which it corresponds under the parallelism (IIp13). The economy of ideas is precisely as closed, necessitated and deterministic as that of bodies.