What is predicate logic and how is it different from propositional logic? Consider the example: All men are mortal. Socrates was a man. Socrates is mortal. This argument can't be expressed as propositional logic. Notice the keyword "All". Propositional logic only has one variable. However the statement "All men are mortal" is a predicate. It is neither true or false. It is both depending on who the subject is also known as the predicate variable. For example, the statement "Elon Musk is a billionaire" is true, but the statement "I am a billionaire" isn't. Unless, I become one in the coming years. Predicates A predicate is a statement that contains variables and that may be true or not depending on the values of these variables. Predicate : P(x) is a color. Predicate Instantiated (Proposition) : P(red) is a color. Predicate's domain (values which x can take): {red, green, apple} You can also think of a predicate as a function. f(x) = ...