Rethinking Spinoza in light of the Principle of Sufficient Reason promises to be important not only for our understanding of Spinoza, but also for our understanding of the philosophical issues Spinoza deals with and that continue to trouble philosophers today.
The purity of Spinoza’s commitment to explanation can best be articulated in terms of his commitments to the Principle of Sufficient Reason (hereafter, the “PSR”) and to his naturalism.
Spinoza can be seen as a pure philosopher, always seeking explanation, always refusing to be satisfied with primitive, inexplicable notions. This purity is most evident in his commitment to the principle that each fact has an explanation, that for each thing that exists there is an explanation that suffices for one to see why that thing exists. Although Spinoza does not himself use the term, this principle is known as the Principle of Sufficient Reason (PSR).
According to Spinoza’s system, all reality flows with strict necessity from the nature of God. With the geometrical method, Spinoza captures the structure of this reality by deducing philosophical conclusions about reality from definitions which express the nature of God.
How many things are there in the world? Spinoza’s answer: one.
Unsurprisingly, then, Spinoza has not solved the mind-body problem. But he has advanced our understanding of it. He has shown how, if one skillfully and consistently wields the PSR and the conceptual barrier between thought and extension, one can construct an argument for the view that there is one substance and one can undermine the Cartesian intuitions that material things and physical things cannot be identical.
As always with Spinoza, it is helpful to begin with God. The system of ideas that are parallel to modes of extension constitutes God’s infinite intellect.
For Spinoza, obviously, the human mind, as well as the human body, are individuals (actually, they are the same individual). However, this individuality of the human mind would be threatened if the human mind were made up of ideas whose contents were relatively disparate, if these ideas were not all focused around a particular unified thing, such as the human body.
Spinoza’s naturalism and rationalism are nowhere more evident and more relevant to contemporary philosophy than in his philosophy of mind. All there is to thought is the having of ideas, representations of certain things. Thus, in laying down requirements on what it is to have an idea or representation of an object, Spinoza is articulating the essence of the mental.
For Spinoza, my mind is simply an idea in God’s intellect; in particular it is God’s idea of my body.