Principle of Duality in the Plane

Theorem
Let $P$ be a theorem of projective geometry proven using the propositions of incidence.

Let $Q$ be the statement created from $P$ by interchanging:
 * $(1): \quad$ the terms point and (straight) line
 * $(2): \quad$ the terms collinear (of points) and concurrent (of lines)
 * $(3): \quad$ the terms lie on and intersect at

and so on.

Then $Q$ is also a theorem of projective geometry.