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A sentence is made of simple propositions, which can be either *true* or *false*. These are called the *truth values* of a propsoition. We do not consider any possibility of *maybe* and the world is presented as black-an-white. For instance, a man is either tall or small, but not medium; he is cleaver or silly, but not ordinary. The contrary of rich is poor, and there is no middle class.
A sentence is made of elementary propositions, connected by the so-called *logical connectors*. The truth value is fixed by the truth values of the components, together with the rules to which the connectors obey.
When we intend to enounce the contrary of a complicated sentence, we have to find a sentence which includes all the possibilities which are not included in the original sentence, leaving no in-between situation.
We detail all this material in the following sections.

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2002-06-10