Why did Denis Diderot believe great actors should not actually feel the emotions they portray?

In his influential essay Paradoxe sur le comédien (The Paradox of Acting), the French philosopher Denis Diderot argued that great actors must possess judgment and penetration without "sensibility"—meaning they should not actually experience the emotions they portray on stage.1

Diderot’s reasoning was rooted in the necessity of consistency and technical control. He observed that actors who rely on their own genuine emotions produce uneven performances, as their playing becomes "alternately strong and feeble, fiery and cold" depending on their personal state of mind. He questioned how an actor could successfully repeat a performance with the same spirit and precision if they were truly overwhelmed by the character's feelings, noting that such actors often become "worn out and cold as marble" by the third performance.2

Ultimately, Diderot believed that acting is a craft requiring a "definite course to the passion" with a clear beginning, middle, and end. By remaining unmoved themselves, actors can maintain the necessary control to deliver a consistent, unified performance that effectively moves the audience every time.2