The inclusion of an item within a theory may be essential or accidental, and if the former then the explanation of its meaning and of its inclusion in the theory cannot be by accidental events and circumstances. Since all events and circumstances – be they social, political, religious, psychological, etc. – are accidental vis-à-vis the ideas they occasion, they cannot serve as explanation of these ideas. The only way to explain the ideas is by showing their essentiality to the theory rather than their importance to the people. Thus, the place of spirits and souls within seventeenth-century science is explained by Platonic ideology.

This demands a concept of nature which is thoroughly efficient causative, and a concept of explanation which is thoroughly informative. This means that theories must explain any given effect by a cause which is separable from it and is conceptually heterogeneous to it. Such a nature and such an explanation are inherently paradoxical and irrational, which explains the preponderance of these traits in seventeenth-century science as created by Kepler, through Galileo and Descartes to Newton.

The primary efficient-causal and explanatory agency is force, which is ontically distinct from matter and from the motion it causes in matter. Soul, spirit and active principle are mere variants of force. Thus, their presence in seventeenth-century science is explained only by their being conceptually essential to any Platonic philosophy of nature.