La Forge was born at La Flèche, where his father was a physician. He also became a physician, but nothing is known about his studies. He spent his life between La Flèche (the town where Descartes had studied at the Jesuit college) and Saumur (in the Loire Valley), where he died in 1666. Around 1650, he was known as a defender of Descartes’ physics. When Clerselier looked for someone to draw the illustrations in order to publish Descartes’ Treatise on Man (a part of The World, published posthumously), as mentioned in the second volume of Descartes’ Letters, La Forge offered to work on these illustrations. La Forge sent not only illustrations but also Remarques to clarify the ambiguities of the Cartesian text while claiming to remain faithful to Descartes.

La Forge's Remarks are footnotes and they often refer to his own forthcoming Traité de l'esprit de l'homme (Treatise on the Human Mind). So, these Remarks are not complete, the longest ones dealing with the structure and functions of the brain and with the “animal spirits,” important topics in Descartes’ text, while others comment on the mechanical explanations given by Descartes. Some of them defend Descartes against the “difficulties” raised by the physician Bartholin (Bartolin) in his Anatomia Reformata. As to the illustrations of the Treatise on Man, La Forge explains that he felt “committed less to representing things according to Nature than to rendering intelligible” what Descartes had to say. For instance, the pineal gland is figured much bigger than it is (see pineal gland, Figure 27).

The Treatise on Man, with Remarks by Louis de La Forge was published in Paris in April 1664, with a preface by Clerselier quoting Saint Augustine (De Trinitate, X, cap. X) and, at the end, a French translation of Schuyl's preface to the Latin translation of the Treatise on Man published in Leiden in 1662 (made from a copy of the French original), dealing with the “bêtes-machines” (see animal) and referring to Saint Augustine (De libero arbitrio, cap. VIII).