1.2. Propositions
A proposition is a declarative sentence that is either true or false. "7 is prime" and "every even number greater than 2 is the sum of two primes" are propositions. "Close the door" and "x is even" are not, the first because it commands rather than states, the second because its truth depends on the unbound variable x.
In Lean, the type Prop classifies propositions.
#check True
We write P, Q, R for propositional variables, which stand for arbitrary propositions.