A rational judgement is one that requires neither an appeal to revelation, nor repeated experimentation. Rather, a rational judgement is one where the proposition is judged according to the law of identity.

To perform a rational judgement, one considers the essence of the subject with respect to the essence of the predicate. Judgement then depends on whether or not an absurdity entails from affirming the relation. When judged according to the law of identity, a proposition is either:

Rationally Necessary: a proposition that does not accept negation in of itself[1]. This is because negating it would violate the law of identity. For example: the proposition “an even number is divisible by two”[2], which must be true, and cannot be false.

Rationally Impossible: a proposition that does not accept affirmation in of itself. This is because affirming it would violate the law of identity. For example: the proposition “an even number is not divisible by two”, which must be false, and cannot be true.

Rationally Possible: a proposition that accepts both affirmation and negation in of itself, because neither entails a violation of the law of identity. For example: the proposition “Zayd will die tomorrow”, which can be true, and can be false.


 

[1] “In of itself” meaning: given the essences of the subject and the predicate.

[2] An even number is (by definition) one that is divisible by two. To negate the divisibility of an even number by two, is therefore a violation of the law of identity. By virtue of the alternative’s absurdity, “an even number is divisible by two” is therefore judged to be rationally necessary.

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