ABSTRACT
This chapter introduces the passages in Spinoza that suggest that he understands principium in the precise sense. Oldenburg freely offers a label for what does the explanatory work in nature, even if it is merely a hypothesis: it is a principle. Turning now to the second class of nontechnical uses, a notable use of principium to describe a primary efficient cause may be found at Descartes's Principles of Philosophy (DPP), D1, where, in offering an example of something related to thought that is not a thought, Spinoza writes that voluntary motion has thought as its principle, although it is not itself a thought. For the understanding of principles in the history of early modern philosophy, Spinoza's use of the term principium shows the variety of its meanings and the danger of taking what looks like a technical term to have the same meaning in different uses.
