It is conjectured that Diderot has, in his work, an established materialist system. In analyzing the parts of such system, it is necessary to keep in mind the idea or hypothesis that runs through it, namely the universal sensitivity of matter expounded in D’Alembert’s Dream (1769). Diderot, assuming that the most rudimentary matter is sensitive, presents three levels of sensitivity: 1) inertia, where sensitivity is characterized by the activity to liberate itself; 2) active, where sensitivity is designated by passivity, or by an unrestrained sensitization; 3) active thinking, where activity and passivity are concomitant. In the human body, the inert state of matter implies the predominance of the brain, while the active state of matter is based on the diaphragm. In the novel The Nun (1770), Diderot demonstrates that matter in a state of inertia is nothing more than contained sensitivity, hence the protagonist is enclosed in a convent, the perfect setting to expose the obstacles that stand against the liberation of sensitivity. In Jacques the Fatalist and his Master (1771-1782), we can see the ascendancy of the diaphragm over the brain, portraying a character who is unrestrained, giving body to the conception of the active state of matter. In another of Diderot’s aesthetic texts, the Paradox of the Actor (1769-1784), the active thinking state of matter is manifested by the brain activity of an actor who gives vent to his sensibility, without the diaphragm taking the reins of his physical complexion.