In his edition of Descartes’ correspondence, Clerselier identifies the addressee of a letter mainly on the Meditations (AT IV 62–65, CSMK 229–30) as a “Mr Buitendyck.” According to Adam and Tannery (AT), this should be Gosuinus (Van) Buitendyck (ca.1585–1661), a minister and curator of the Latin School at Dordrecht. However, in the first, albeit partial, publication of the letter by Tobias Andreae (1604–76), the addressee is qualified as iuvenis ornatissimus – a title given to students. So the addressee of Descartes’ letter should be someone who was student between 1642 and 1650. This rules out Gosuinus. The most serious candidate is Petrus Buitendyck who, on February 6, 1644, at nineteen years old, enrolled as a student of theology at Leiden. He is probably the same Petrus Buitendyck who again enrolled as a theology student at Leiden on November 27, 1645, at twenty years old – this time described as being from Dordrecht and the son of Gosuinus, the minister. The same Petrus presumably pops up again as a student of theology at Franeker in 1647. A less likely candidate is Samuel à Buittendich from Dordrecht, who on June 21, 1647, at the age of twenty, enrolled at Leiden and is probably Petrus's younger brother – less likely because his reference would be the Principles (1644) rather than the Meditations (1641–42). However, there is no absolute certainty.

Little is known about Petrus's later career, except that in 1658 he became minister in Nieuw-Beijerland (a small village in South-Holland). Descartes’ letter, which, if the addressee is Petrus, should be dated 1644 or later, provides an answer to three questions: whether it is allowed to doubt the existence of God, whether it is evil to suppose something false with respect to God, and whether the soul of animals consists in motion. In his answer, Descartes makes a few restrictions that are not found elsewhere. It is permitted to doubt the existence of God, that is, not to be certain of it, as long as this “doubt” is purely intellectual. And, of course, we must not suppose anything false about God, but an evil genius (which is the real object of the question) is not the true God but an idol. Instead of identifying the animal soul with motion, Descartes would rather side with scripture (Deut 12:23) and say that it resides in the animals’ blood.

See also Animal, Doubt, Existence, God