“After experience had taught me that everything that frequently happens in common life is vain and futile, I saw that everything which was for me a cause or object of fear had in itself nothing good or bad, except to the extent that the soul was agitated by it, I finally decided to seek if there was something which was a true good, capable of being communicated and by which alone, all other things having been rejected, the soul would be affected; nay more, if there were something the discovery and acquisition of which would enable me to enjoy continuous and supreme joy for eternity.” Left unfinished, the Treatise on the Reform of the Understanding nonetheless constitutes a crucial work for understanding the evolution of Spinozist thought. By analyzing the true idea given, the rules of a true definition, the properties of the understanding and the rigorous respect for the deductive order, it provides the essential intellectual instruments which will be implemented in the development of Ethics, with the exemplary rigor and mastery that we know.