The seventeenth-century doctrine known as occasionalism arose in response to a perceived problem. Cartesian philosophy generated the problem and provided the context for the answer. In the Cartesian ontology, mind and matter are substances totally different in nature. Souls or minds have modes of thought but not modes of extension; bodies have modes of extension but not of thought. Modes are properties that affect or modify substances. A substance with a particular mode can be conceived as not having this mode, but the mode cannot be conceived apart from the particular substance of which it is the mode. The modes of each substance belong to that substance alone and cannot belong to any other substance. 1 Each mind has its own thoughts, that is, its own perceptions and volitions, and they are numerically distinct from the thoughts of every other mind. Likewise, each body has its own figure, and each moving body has its own motion. Even when two bodies are said to have the same shape, the mode which is the figure of one body is numerically distinct from the mode which is the figure of the other.
In the 1640s, the following question was put to Descartes by Pierre Gassendi and again by Princess Elizabeth of Bohemia: how can the human mind act on the human body, and the body on the mind, if they are two substances totally different in nature? Descartes responds to Gassendi by dismissing the question:
The whole problem contained in such questions arises simply from a supposition that is false and cannot in any way be proved, namely that, if the soul and the body are two substances whose nature is different, this prevents them from being able to act on each other. 2
To Elizabeth, he acknowledges that the question is a fair one. He appeals to the notion of the union of soul and body, ‘on which depends