How did Leon Battista Alberti explain the relationship between a painting and a window?

In his influential 1435 treatise Della pittura, Leon Battista Alberti conceptualized the painting as a "window to the world" to explain the principles of linear perspective.12

Alberti instructed the painter to imagine viewing a scene through a window using only one eye and keeping the head perfectly still. In this model, the canvas is treated as a transparent plane—a "decorated window"—that intersects the visual pyramid of rays proceeding from the objects in the scene to the observer's eye. By projecting the scene onto this vertical plane, the artist creates a rational, mathematical space where all elements bear a consistent relationship to one another and to the observer.32

This system allows the painting to function as an extension of the real world, where the observer feels as though they are looking through an opening into a three-dimensional space rather than at a flat surface. By controlling the observer's distance and height, Alberti’s method ensures that the microcosm of the painting and the observer's reality become visually unified.12