Is logical fallacy different from logically false?

I believe that by definition yes, but perceive that every day language often suggests that logical fallacy means logically false. That's, if something contains fallacious reasoning, then it's also false.

By def. logical fallacy means "loose reasoning", which doesn't follow "truth evaluation" properly. That is, it might do derivations, which are not "valid", between logical steps.

Then the use of logical fallacies in this contexts also skews the view on their meaning. Because then Ad Hominem could be used to make argument false, even when as just a fallacy, it merely suggests that the reasoning might have "deviated" from the context that it should have stayed in. That is, that "you're ugly and fat, therefore your argument is false" is fallacious.

However, if one postulates that "arguments of ugly and fat are false", then certainly it's a true statement by definition. And there's no particular reason, that I can see, why such premise could not be taken. Since it's about as "valid" as "arguments of ugly and fat are true" or "arguments of ugly and fat behave like all other arguments".


  • What do you mean with "logically false" : contradictory ? or simply false. Commented Jun 5, 2018 at 9:21
  • @MauroALLEGRANZA False by application of logical rules. I used it to emphasize that it's not some "heuristic false", but that which applying exact logical rules would give out. Sometimes people claim something is "logically false", even when it's not logically false, but heuristically false (that they think it's false). One should also note that not everything is interpretable in logic. So claiming logical truth to a thing that logic cannot model exactly, contains error.
    – mavavilj
    Commented Jun 5, 2018 at 9:22
  • A fallacy is usually a falalcious argument. A formal fallacy is an invalid argument, i.e. an argument that could have true premises, but still have a false conclusion. Commented Jun 5, 2018 at 9:24
  • A Logical truth is not a truth only, but "is a statement which is true, and remains true under all reinterpretations of its components other than its logical constants." Thus, the negation of a logical truth is always false, i.e. contradictory. Commented Jun 5, 2018 at 9:26
  • Having said that, if you equate "logically true" with valid and "logically false" with invalid, then NO: a formal fallacy is invalid, and thus "logically false". Commented Jun 5, 2018 at 9:30

1 Answer 1


A logical fallacy occurs when the premises of an argument do not logically imply the conclusion. The following is an example of fallacious logic :

If p, then q


Therefore :


This is the fallacy of affirming the consequent. If p then q may be true, and q by be true but q need not be true by virtue of the truth of p. Compare : If it is raining, then the pavements are wet; the pavements are wet; therefore it is raining. But the pavements could be wet because of a broken drain or a truck spillage.

'Logically false' is not a standard phrase, as Mauro Allegranza indicates. I think you may have in mind something such as :

'A • ∼ A' = 'A and not-A'. Assuming bivalence (that every proposition is either true or false - there is no third value) and that 'A' has the same sense and reference in both occurrences, this is a logical falsity. It is self-contradictory and there are no conditions in which it can be true. It is not a fallacy because it involves no argument. 'Logical falsity' is not an unreasonable name for it but I recommend the sharper specificity of calling it a self-contradiction or a self-contradictory proposition.

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