Around 1420 Filippo Brunelleschi stood in a Florence doorway and painted the Baptistery so exactly that, viewed through a peephole against a mirror, paint and building were indistinguishable. He had found the rule: parallel lines running away from you appear to meet at a single vanishing point.
Leon Battista Alberti wrote it down as the costruzione legittima — draw the floor's edge, run orthogonals from it to one point, and space the depth lines by a fixed geometric ratio. No measuring; the depth is pure construction.
That single point sits on the horizon, which is nothing but your own eye level. Every standing figure your height has its eyes on that same line — which is why the horizon slices them all at the eyes, near or far.
Split the convergence toward two points, or lift the horizon to the ceiling, and the geometry stops agreeing with itself — the floor fractures and the depth dissolves back into flat paper. The illusion is only ever as strong as its one point.
The frame slipped mid-construction. The lesson continues without it — nothing you were exploring is lost.