The proof uses modal logic, which distinguishes between necessary truths and contingent truths.
A truth is necessary if its negation entails a contradiction, such as 2 + 2 = 4; by contrast, a truth is contingent if it just happens to be the case, for instance, "more than half of the earth is covered by water". In the most common interpretation of modal logic, one considers "all possible worlds". If a statement is true in all possible worlds, then it is a necessary truth. If a statement happens to be true in our world, but is false in some other worlds, then it is a contingent truth. A statement that is true in some world (not necessarily our own) is called a possible truth.
A property assigns to each object, in every possible world, a truth value (either true or false). Note that not all worlds have the same objects: some objects exist in some worlds and not in others. A property has only to assign truth values to those objects that exist in a particular world. As an example, consider the property
- P(s) = s is pink
and consider the object
- s = my shirt
In our world, P(s) is true because my shirt happens to be pink; in some other world, P(s) is false, while in still some other world, P(s) wouldn't make sense because my shirt doesn't exist there.
We say that the property P entails the property Q, if any object in any world that has the property P in that world also has the property Q in that same world. For example, the property
- P(x) = x is taller than 2 meters
entails the property
- Q(x) = x is taller than 1 meter.
The proof can summarized as:
- IF it is possible for a rational omniscient being to exist THEN necessarily a rational omniscient being exists.