Now define sets A and B from universe X (universe X is isomorphic with universe
U), where these sets might represent linguistic ideas or thoughts. A propositional calculus
(sometimes called the algebra of propositions) will exist for the case where proposition P
measures the truth of the statement that an element, x, from the universe X is contained
in set A and the truth of the statement Q that this element, x, is contained in set B, or
more conventionally,