Jacques Rohault

Life and Intellectual Context

Jacques Rohault (1618–1672) was a French philosopher and natural scientist associated with Cartesianism, the philosophical system developed by René Descartes. Born in Amiens, he was educated in Jesuit colleges, where he received a thorough grounding in Scholastic Aristotelian philosophy. Like many of his contemporaries, he later turned away from Aristotelian natural philosophy toward new mechanistic and mathematical approaches to nature.

By the middle of the 17th century, Paris had become a major center for the discussion of Descartes’ ideas. Rohault settled there and joined circles of intellectuals interested in the emerging mechanical philosophy, which explained natural phenomena exclusively by the size, shape, motion, and arrangement of particles of matter. His Parisian home became well known as a venue for private lectures and demonstrations of physical experiments.

Rohault lived in the generation after Descartes and helped shape how Cartesian philosophy was understood and taught. While not an original metaphysician on the scale of Descartes or Malebranche, he played a crucial mediating role: he presented Cartesianism in a clear, systematic way and aligned it with contemporary experimental practices, making it accessible to students and non‑specialists.

He died in Paris in 1672, at the height of his reputation as a leading public interpreter of Cartesian physics.

Works and Teaching

Rohault’s main work is the Traité de physique (Treatise on Physics), first published in 1671. Intended as a comprehensive textbook, it quickly became one of the most widely used expositions of Cartesian natural philosophy. It covers topics such as:

• the nature of body and extension
• the laws of motion and impact
• gravity and the behavior of terrestrial bodies
• fluids, air pressure, and vacuum experiments
• the structure of the cosmos, including the Cartesian vortex theory

The Traité is notable for its pedagogical structure. Rohault begins from basic Cartesian principles about matter and motion, then proceeds systematically to phenomena: falling bodies, pendulums, magnetism, optics, and cosmology. He integrates discussion of contemporary experiments, including those associated with Torricelli, Pascal, and Boyle, treating them not as isolated curiosities but as confirmations or tests of mechanical explanations.

In addition to the Traité de physique, Rohault wrote a Traité de métaphysique, published posthumously (1679). This shorter work follows Descartes’ main lines: the method of doubt, the existence of God, the distinction between mind and body, and the foundations of knowledge. It reinforces Rohault’s reputation as a faithful, if less speculative, Cartesian.

Rohault was also influential as a teacher and lecturer. His private courses in Paris drew students from the nobility and the educated public interested in the new science. Accounts from contemporaries describe how he used experimental apparatus—air pumps, pendulums, lenses—to illustrate Cartesian principles, thereby helping to associate Cartesianism with hands‑on empirical inquiry rather than purely armchair speculation.

Philosophical and Scientific Views

Rohault’s philosophy is best understood as a refined exposition of mechanical Cartesianism.

1. Matter and Motion
Rohault adopted the Cartesian view that the essence of body is extension in length, breadth, and depth. Bodies are differentiated only by their size, shape, and motion; there are no Aristotelian substantial forms or real qualities such as heaviness or heat. Accordingly, he explained:

• heaviness as a result of pressure and vortex motions of subtle matter
• heat as a mode of motion of small particles
• light as a mechanical propagation in an all‑pervading medium

These accounts aligned with Descartes’ own, but Rohault often tried to present them in a more didactic and experimentally oriented fashion.

2. Laws of Nature and Impact
Rohault accepted Descartes’ laws of nature, conceived as general rules God imposes on matter in motion. He was especially concerned with the laws of collision between bodies. Like many Cartesians, he discussed idealized impact problems—two bodies colliding head‑on, or glancing blows—to illustrate how conservation‑like principles might hold in a mechanical universe.

Later critics, especially Leibniz and Newtonian physicists, found faults in Cartesian collision laws. Nonetheless, Rohault’s clear treatment made these debates more accessible and helped structure 17th‑century discussions about conservation of motion and force.

3. Experiment and the Mechanical Philosophy
Unlike some caricatures of early Cartesians as purely speculative, Rohault took experiment seriously. He described and repeated many of the era’s most famous experiments, including those showing the existence of a vacuum and the weight of air. However, he tended to treat experimental results as illustrations or tests within an already‑accepted mechanical framework, not as independent sources of radically new theories.

Proponents of Rohault’s approach emphasize that he contributed to normalizing a practice in which mechanical hypotheses were checked and refined by experiment. Critics argue that his framework left little room for reconsidering fundamental assumptions when experiments conflicted with Cartesian principles.

4. Metaphysics and Theology
In metaphysics, Rohault largely followed Descartes:

• the mind–body distinction: mind as a thinking, non‑extended substance; body as extended, non‑thinking substance
• the existence of God as a perfect being guaranteeing the reliability of clear and distinct ideas
• the rejection of Scholastic forms and occult qualities

Rohault generally avoided the more intricate theological and occasionalist questions that occupied other Cartesians (such as Malebranche’s theory of vision in God). His aim was to provide a stable metaphysical underpinning for the mechanical explanation of nature, while not overcomplicating matters for students.

Reception and Legacy

Rohault’s immediate influence was substantial. In late 17th‑century France, his Traité de physique became a standard textbook and an important vehicle for transmitting Cartesian physics. It helped to fix a relatively uniform version of Cartesian natural philosophy in the minds of students and readers across Europe.

One striking aspect of his legacy is the way his work became a bridge between Cartesianism and Newtonianism:

• In the early 18th century, the English theologian and philosopher Samuel Clarke produced a widely read Latin translation of Rohault’s Traité de physique.
• Clarke added extensive Newtonian notes, in which he corrected or replaced Cartesian explanations with Newtonian ones—for example, revising accounts of gravity, planetary motion, and collision.
• As a result, the book functioned for some time as an introduction to Newtonian physics structured around a Cartesian text, illustrating a broader intellectual transition from one scientific framework to another.

Within French Cartesianism, Rohault’s work also influenced Pierre‑Sylvain Régis, whose own Système de philosophie drew on Rohault’s clear expository style. Rohault’s relatively cautious and didactic metaphysics contributed to what some historians call the “textbook Cartesianism” of the late 17th century: a codified, systematic, and sometimes simplified version of Descartes’ thought.

Historians of philosophy and science often view Rohault less as an original theorist than as a codifier, popularizer, and experimental pedagogue. Proponents note that without such figures the new mechanical philosophies might have remained confined to small elite circles. Critics suggest that the textbook tradition sometimes hardened Descartes’ ideas into a rigid doctrine, making it harder to adapt to new developments such as Newton’s gravitation theory.

Despite these debates, Rohault is widely recognized as an important intermediary in the Scientific Revolution: a figure who helped to consolidate, teach, and gradually transform the mechanistic worldview that dominated much of 17th‑century European natural philosophy.