The ethics; Treatise on the emendation of the intellect; Selected letters Book

Spinoza is the most systematic and reasoned thinker of the Rationalists. His ethics is a huge set of definitions, axioms, propositions, proofs, and corollaries in a bizarre format called the "geometrical" style, loosely based on the very rigid and sequential nature of mathematical proofs. His systematic approach leads him to some very unusual conclusions about the nature of God, the human being and its relation to the world, and his ultimate ethical imperatives, but it is nonetheless a very original and interesting train of thought, and a much bolder and more striking set of ideas than those posited by the other most well-known Rationalists (Descartes and Leibniz).