Talk:Modal fallacy

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The part about the president being over 35 years old is completely unclear. Can someone knowledgeable in modal logics improve it?

Feel free to tell me otherwise (I am not an expert on modal logic), but as stated it seems that the argument is completely valid. If we are in a world where the President must be 35 years old and Donald Trump is the President, then Donald Trump must be over 35 years old. The article seems to assume that just because Donald Trump is not always 35 years of age or older, that means that the argument is invalid, but certainly by the argument's own premises, in this world Donald Trump would not be President and hence the argument would still be valid. Is there some nuance of modal logic here that isn't being expressed? Tyg13 (talk) 18:36, 21 November 2017 (UTC)[reply]

The key here is the word "necessarily." By concluding that Donald Trump is necessarily 35 years or older, the argument claims that Donald Trump has to be over 35 years, even in a world (not our own) where he does not exist. In modal logic, truths can be divided into two types: necessity and contingency. Truths by necessity include: "If a statement is true, then it is true" which has to be true no matter which world you live in. On the other hand, contingent truths do not have to be true in all possible worlds. One cannot conclude a truth by necessity with premises based on contingency. That is the fallacy. — Preceding unsigned comment added by 103.220.77.90 (talk) 05:34, 15 January 2018 (UTC)[reply]