Lately I have come across a person answering the other person making an argument by calling them crazy and insane. Not even going into their argument and not using any manipulative tactics. Simply calling the person insane. What is the name of this logical fallacy? Is it a form of ad hominem? I've seen it somewhere before but I haven't found it when searching for it. From the description of it it didn't fit the definition of the "appeal to ridicule" fallacy. In case anyone knows I'd be very thankful. :)
According to Michel Foucault, calling someone insane is a classical example for ordering discourses by excluding opinions that are perceived too far outside the present rational discourse options, but which are not directly forbidden (like denial of Holocaust in many countries).
A society limits the things they discuss about. You would not even discuss killing babies for fun, or re-opening concentration camps. Instead of discussing those arguments, people opt for calling the argument "insane", thereby eliminating it from the discourse.
This is a common, and useful, procedure, but of course it can be taken advantage of. The accusation may be wrong, or the notion of insanity changes over time. (400 years ago, it might be seen as insane not to believe in god. Today, many think it is a proof of insanity if you actually believe in god.)
Thus, calling an argument (or the one presenting it) insane is a technique to exclude the argument from any discussion. It is not a logical fallacy, but a rejection that may be anything from well-founded to outright incorrect.
In a broader sense, it can be seen as an ad hominem argument, but with the restriction that it is itself not designed as an argument, but as a means to end discussion.
Calling the other guy insane, is of course Poisoning the well, the worst kind of all the ad hominem attacks. The first English usage from 1864:
...I am at war with him; but there is such a thing as legitimate warfare: war has its laws; there are things which may fairly be done, and things which may not be done. I say it with shame and with stern sorrow;—he has attempted a great transgression; he has attempted (as I may call it) to poison the wells. .... my present subject is my Accuser; what I insist upon here is this unmanly attempt of his, in his concluding pages, to cut the ground from under my feet;—to poison by anticipation the public mind against me, John Henry Newman, and to infuse into the imaginations of my readers, suspicion and mistrust of everything that I may say in reply to him. This I call poisoning the wells. -- John Henry Newman
This could also be an Appeal to Motive which criticizes the objectivity of the person making the argument by calling into question his or her motivations for favoring an argument.
If only the insane favor an argument, all people who favor this argument must be insane.
My opponent is making this argument.
My opponent is insane.
Broadly this is an Ad Hominem attack that is circumstantial (My opponent has taken this stance in the past, therefor he is unqualified to discuss this issue.)
As Yannis's comment noted, mainly it is a version of Ad hominem fallacy
Ad hominem (Latin for "to the man" or "to the person"), short for argumentum ad hominem, is a fallacious argumentative strategy whereby an argument is rebutted by attacking the character, motive, or other attribute of the person making the argument, or persons associated with the argument, rather than attacking the substance of the argument itself.
Specifically, it's a combination of "Appeal to Motive" and even more so, "Appeal to identity" (both of which are a version of Bulverism, which is a fallacy of assuming the argument to be invalid and then offering an explanation of why it's invalid centered on who posed the argument).
Among most famous political uses of this approach, taken to the extreme, was Soviet Union, where people with wrong political ideas were declared to be mentally ill, literally, and incarcerated in mental institutions.
A more recent but not yet quite as extreme approach seems to be popular in USA lately ("Psychology Today", "Psychology Today" again, "Salon", "Daily Kos" for proponents; and "Reason" and "National Review" for criticisms of the idea).
It can also in certain cases be seen as an instance of The Appeal to Nature fallacy.
The implication being made is that "natural", non-crazy, thinking, refutes the proposition.