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).

Spinoza (2008) - One: Spinoza's Understanding and Understanding Spinoza