(closed sentence) is a sentence that is either true or false, but not both. We can denote each statement as a letter. For example,
: 1 cm is exactly equal to 10 mm
If we can determine the truth or falsity of a statement, then it has a truth value
An open sentence
is a sentence that has a variable. The truth value depends on that variable.
(or replacement set) is a list of elements that can be used to replace the variable.
The truth set
is the list of elements from the domain that makes the open sentence true.