 A third reason to prefer the standard definition in philosophy is that it makes the definitions of “atheism” and “theism” symmetrical. One problem with defining “atheism” as a psychological state is that philosophers do not define “theism” as a psychological state, nor should they. “Theism,” like most other philosophical “-isms”, is understood in philosophy to be a proposition. This is crucial because philosophers want to say that theism is true or false and, most importantly, to construct or evaluate arguments for theism. Psychological states cannot be true or false, nor can they be the conclusions of arguments. Granted, philosophers sometimes define “theism” as “the belief that God exists” and it makes sense to argue for a belief and to say that a belief is true or false, but here “belief” means “something believed”. It refers to the propositional content of belief, not to the attitude or psychological state of believing. If, however, “theism” is defined as the proposition that God exists and “theist” as someone who believes that proposition, then it makes sense to define “atheism” and “atheist” in an analogous way. This means, first, defining “atheism” as a proposition or position so that it can be true or false and can be the conclusion of an argument and, second, defining “atheist” as someone who believes that proposition. Since it is also natural to define “atheism” in terms of theism, it follows that, in the absence of good reasons to do otherwise, it is best for philosophers to understand the “a-” in “atheism” as negation instead of absence, as “not” instead of “without”—in other words, to take atheism to be the contradictory of theism.