What is a proposition?

An assertion is a statement.

A proposition is an assertion which is either true or false (but not both).

The Following are propositions:

• 4 is a prime number
• 3 + 3 = 6
• The moon is made of cheese.

The following are not propositions:

• x+y > 4.  Is this true or false? It depends on the value of x and y, the statement takes a true or false value.
• x=3. You cannot associate a truth value to this because it simply assigns a value to x.
• Are you leaving? This is not an assertion, it is a question.
• Buy 4 Books This is not an assertion, it is an order.