In a word: duh. However, I do not for a second think that you would not listen more closely to someone like Stephen Hawking than you would a guy shouting "Praise Jesus" on a street corner. There is no fallacy involved here. If Hawking is speaking nonsense, his being Stephen Hawking is not going to save him.
-- The famous "reverse" Ad-Hominem attack? WAT
TLDR: Famously intelligent and thoughful people are frequently full of shit, evil, or amoral. Same rules apply. [1]
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[1] Its worth noting [a] ad-hominem is not a per-se fallacy; And [b] its useful to questions motives. But since many serial killers, terrorists, and supporters of the hollocaust could be considered "famously intelligent and thoughtful people", it goes without saying that arguments should be evaluated on the merits.