Rene Descartes and Baruch Spinoza were 17th century philosophers who debated the nature of reality and the relationship between mind and body. Descartes believed in dualism - that the mind and body are two distinct substances. Spinoza argued for monism, that mind and body are one substance that is dependent on God. While they disagreed on this key metaphysical point, both philosophers believed that mathematics was the best way to understand truth and that human actions are determined by preceding causes.