Principle of Duality in Space

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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 plane
$(2) \quad$ the terms lie on and intersect at

and so on.

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


Proof



Sources