Recent Cartesian scholarship has increasingly been inclining toward the view that Descartes’ ethical writings in his later years deserve recognition as a virtue ethics. While the earlier morale par provision in the Discourse on Method, part III, lays the foundations for what would later evolve into Descartes’ developed ethics, his full-fledged account of virtue would appear only in the mature ethical writings (notably his 1645–49 correspondence and the Passions of the Soul[1649]).

Descartes does not disguise the immense influence of the ancient ethicists on his thinking, especially the Stoics. Yet he recurrently conveys his discontent with the Aristotelian, Stoic, and Epicurean treatments of key traditional notions such as “virtue,” “generosity,” “supreme good,” and “happiness” and typically imbues them with new meaning. In a letter to Princess Elisabeth of August 4, 1645, Descartes presents his own definition of “virtue” (la vertu) as “a firm and constant resolution to carry out whatever reason recommends without being diverted by [one's] passions or appetites” (AT IV 265, CSMK 257–58). Stressing the novelty of his definition, he adds: “Virtue, I believe, consists precisely in sticking firmly to this resolution; though I do not know that anyone has ever so described it. Instead, they have divided it into different species to which they have given various names, because of the various objects to which it applies” (AT IV 265, CSMK 258).

This remark alludes to a paramount feature of Descartes’ account of virtue and ethical reasoning, which may be called the “unity of virtue.” Rather than providing a comprehensive set of ideals of character or first-order rules for action to be followed in the various circumstances of our lives, Descartes places at the center of his mature ethical system a single, second-order duty to practice virtue, which he identifies with the good use of the will. In the dedicatory letter to Elisabeth prefacing the French edition of the Principles, Descartes writes that all the particular virtues – justice, courage, temperance, and the like – proceed from this single merit of possessing a “firm and powerful resolve always to use [one's] reasoning powers correctly, as far as he can, and to carry out whatever he knows to be best” (AT VIIIA 1–3, CSM I 191).