The Principle of Duality
See also: Duality (order theory)The above propositions display the following interesting pattern. Each of the identities stated above is one of a pair of identities such that each can be transformed into the other by interchanging ∪ and ∩, and also Ø and U.
These are examples of an extremely important and powerful property of set algebra, namely, the principle of duality for sets, which asserts that for any true statement about sets, the dual statement obtained by interchanging unions and intersections, interchanging U and Ø and reversing inclusions is also true. A statement is said to be self-dual if it is equal to its own dual.
Read more about this topic: Algebra Of Sets
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“We seem to be pariahs alike in the visible and the invisible world, with no foothold anywhere, though by every principle of government and religion we should have an equal place on this planet.”
—Elizabeth Cady Stanton (1815–1902)