What is Nicolas Poussin’s theory of the modes in painting?

Nicolas Poussin’s "theory of the modes," outlined in 1647, was inspired by the musical modes of the ancient world. Poussin proposed that every element of a painting—including line, form, and color—should be deliberately selected to evoke an emotional response in the viewer that is appropriate to the subject matter.1

Under this theory, the artist adjusts the visual components to match the theme:
* Severe themes: Should be rendered with grave, austere qualities.1
* Joyous themes: Should be depicted in an uplifting manner.1

Poussin applied this principle by utilizing specific color harmonies to influence the viewer's mood, such as employing discordant harmonies for tragic subjects and more seductive, lyrical palettes for tender themes. This concept was highly influential, as it established the idea that the fundamental rudiments of painting are inherently capable of arousing emotion, a notion that became foundational to the later evolution of abstract art.1