“This issue is not whether Donald Trump is mentally ill but whether he’s dangerous."
- James Gilligan, speaking to a conference of Trump-hating psychiatrists at Yale*
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This is my new favorite example of the logical fallacy called petitio principii (aka begging the question).
"Argument is supposed to appeal to things which are known or accepted, in order that things which are not yet known or accepted may become so," writes Professor of Logic Madsen Pirie. "The fallacy of the petitio principii lies in its dependence on the unestablished conclusion. Its conclusion is used, albeit often in a disguised form, in the premises which support it."
No disguise in this case. Gilligan is overtly begging us to accept his unestablished conclusion and move on to an even more tenuous proposition.
This quote also exhibits qualities of the plurium interrogationum (lit. many questions) fallacy. Professor Pirie calls this "the complex question" fallacy.
"When several questions are combined into one, in such a way that a yes-or-no answer is required," he writes, "the person they are asked of has no chance to give separate replies to each, and the fallacy of the complex question is committed."
* "Shrinks Define Dangers of Trump Presidency," Psychology Today, 4/20/2017
