What is fallacious about an argument that "I know X well, so she is right"?

Question

I often hear from media personalities that they are right about a theory or assertion merely by associating themselves with someone or something.

For example:

• "I know Mr. X very well (for years) and i know that he wouldn't do something like that."
• "I've been in the X Organization for years. The X Organization would clearly do X, Y, Z"

What is this fallacy called? I tried reviewing the list of fallacies on the internet and couldn't figure it out.

Or is this not really categorized as a fallacy?

Answer 1

How is this different from argument from authority? In this case, the person claims authority via long-term association. But the falsehood is not that long-term association makes you familiar with something; the falsehood is that because you are an authority on X that you always know how X will act.

Answer 2

I would go with Fallacy of Composition.

The fallacy of composition arises when one infers that something is true of the whole from the fact that it is true of some part of the whole (or even of every proper part). For example: "This fragment of metal cannot be broken with a hammer, therefore the machine of which it is a part cannot be broken with a hammer." This is clearly fallacious, because many machines can be broken into their constituent parts without any of those parts being breakable.

This fallacy is often confused with the fallacy of hasty generalization, in which an unwarranted inference is made from a statement about a sample to a statement about the population from which it is drawn.

It is kind of the reverse of the norm because we are infering: because it is true of some part of the whole, it is true of the whole. But it is still a compositional fallacy.