Renaissance
Painting should obey the same logic as the real world, built on math, anatomy, and the dignity of the human being.
The Renaissance was a reaction against the flat, golden, symbolic art of the medieval church, where figures floated in heaven with no weight and no shadow. Artists in Florence and beyond decided the world had structure you could measure. They cracked linear perspective, so a painted floor receded into a believable distance, and they studied corpses to learn how a body holds itself up.
The result was a kind of confidence that still feels modern. People in these paintings stand in real space, lit by real light, thinking real thoughts. It mattered because it put the human being at the center of the picture and insisted that observation, not just doctrine, could tell you the truth.