Eustachius studied philosophy and theology in Paris, obtained a “license” in 1604, and taught philosophy at the Collège de Calvi, an arts college associated with the Sorbonne. In 1606 he entered the Feuillants, a Cistercian order, and became involved in the education of the members of the order. Eustachius was the author of the Summa philosophiae quadripartita (1609), an influential survey of late Scholastic Aristotelianism. Its four parts cover the main areas of philosophy: logic, ethics, natural philosophy, and metaphysics (see Ariew et al. 1998, 69–71). In a letter to Mersenne from 1640, Descartes remarked that he had bought a copy of this work, “the best book of its kind ever made” (AT III 232, CSMK 156). He intended to print it with critical comments to compare traditional Aristotelianism with his own philosophical program, but he never carried out this plan. The reasons are unclear. Perhaps he feared that an open attack on Scholastic philosophy would upset traditional philosophers, especially those teaching in the universities, and prevent them from approving his Meditations. Perhaps he was busy with other projects in subsequent years, in particular with his extensive correspondence and his replies to the objections written against the Meditations. Or perhaps he abandoned the project because he was unable to secure Eustachius's permission, since the latter died in 1640 (see AT III 260, CSMK 161).

The Summa, which combines elements from Thomism and Scotism, contains many traditional doctrines, among them syllogistic logic, hylomorphism, an account of the four Aristotelian causes, and a theory of categories. It is hardly surprising that Descartes chose it as the target of his critique because it provided a much shorter account of traditional Aristotelianism than the long Jesuit commentaries, known as the “Conimbricenses,” or the extensive treatises by Franciscus Toletus and Antonius Rubius (Gilson 1913, Ariew 2011). He believed that anyone who had the chance to read his comments on this book would easily see its deficits and “learn to scorn it at the same time” (AT III 259–60, CSMK 161).

Despite this harsh critique, there are striking similarities between the Scholastic doctrines expressed by Eustachius and Descartes’ own opinions (Van der Pitte 1988). The most obvious parallel is in the theory of ideas (Perler 1996).