Appeal to belief

The logical fallacy of appeal to belief is committed when an argument infers the truth (or plausibility) of a proposition merely from the fact that it is widely believed. The fallacy is most commonly found in arguments over ethics:

Most Americans hold that the Vietnam War was morally indefensible. Therefore, the Vietnam War was morally indefensible.
Southern segregationists didn't see anything wrong with what they were doing. Therefore, it must not have been wrong. (or: Therefore, it wasn't wrong to them.)

The fallacy is much more rarely committed in matters of positive science, since relatively few people would fail to see the fallacy in arguments such as:

Just about everyone in Ptolemy's day was convinced that the Sun travelled around the Earth. Therefore, the Sun travelled around the Earth in Ptolemy's day.

There are a few exceptions to this rule--specifically, it is likely to be committed when trying to convince a person that certain widely unpopular beliefs are false. For example:

Throughout history, most people have believed in some sort of God. Therefore, you should also believe in God.

It's worth noting that the argument from belief is closely related to certain other logical fallacies that involve a confusion between justification and widespread belief amongst a given group of people. If the group whose beliefs are appealed to is a group of putative experts, then the argument from belief takes on the form of an appeal to authority; if the group whose beliefs are appealed to is a group of respected elders or the members of one's community over a long period of time, then the argument from belief takes on the form of an appeal to tradition.

Further, as with appeals to authority or to tradition, there are some limited circumstances in which an argument from belief may not be a fallacy. For example, when the subject being considered are the results of democratic processes, points of etiquette, or other matters of social convention, arguments of the following form may be valid:

Most people in Russia think that it is polite for men to kiss each other in greeting. Therefore, it is polite for men to kiss each other in greeting in Russia.
Nearly all Americans think that you should drive on the right side of the road. Therefore, you should drive on the right side of the road in the United States.