The logical fallacy of insisting on information that isn't required to determine the truth of something.

I don't know if this is a fallacy or not. For example, in the case of a murder trial, we know a murder has been committed and we may even know that the suspect is guilty. But in a court of law we also have to determine a motive in order to carry out sentencing. There is a difference between murder in the first degree, second-degree murder and manslaughter. All of these have to do with intent and not about whether the murder was carried out or not, or whether the person is guilty or innocent, but what was their motive or intent.   

OK, well, maybe motive wasn't a good example in terms of insisting on information required although if we put aside classification of the crime and just stick to determining who was responsible then I think it's fair to say we don't necessarily need motive.

Let's just think of the fallacy in general terms where there is insistence on information to determine the truth of a situation which isn't required because there is other information that makes it possible to determine the truth.

This is an example of an appeal to evidence or examples that are not relevant to the argument at hand. It commonly appears as a last resort when evidence or rational arguments fail to convince the listener. Look up the term fallacies of relevance,  which appeal to evidence or examples that are not relevant to the argument at hand.

Thanks, Shawn. I'm not sure one of those fallacies really fits. The thing is, it's not that the information isn't relevant exactly, it's that it doesn't need to be known because there's sufficient evidence to prove the case without it. We would expect the information to be there, just that it doesn't need to be found or explained.

While I do believe in many conspiracies, one event I believe to be as reported are the moon landings, an astonishing achievement. Moon hoaxers constantly demand that this or that piece of information needs to be known or explained but if the evidence provided is sufficient to make the case then all the pieces of the jigsaw don't need to be known because the jigsaw pieces available are sufficient. If, for example, certain pieces weren't available then it might be reasonable to demand the unknown pieces be found, for example, if there weren't masses of pieces of visual and audio evidence from the moon landings then certain information might need to be put forward to convince people they happened but as there are masses of pieces of visual and audio evidence, other information isn't necessarily required whether it's available or not.

Essentially, what moon hoaxers do is focus on elements of space they know very little about and make lots of assertions that this that or the other is impossible while ignoring the very clear artefactual evidence or they claim the artefactual evidence isn't genuine when they cannot put forward a good argument to make that case.

[To Petra Liverani]

Actually, I believe you're right, Shawn, as explained by RotE and mentioned by Destone, the red herring fallacy (relevance category) can apply here as can the non sequitur.

With regard to your Example 2, since the issue is whether A committed homicide (and not just whether the deed of killing was done), intent is an element of homicide that must be proven.  Although Sally is correct that motive is not required to prove homicide, intent is.  Motive can and often is evidence of intent.  Therefore, although John’s last statement is legally incorrect, talking about motive is not a red herring since it would be relevant to the issue of whether a homicide was committed.   Even if A’s intent could be proven without motive, proving motive would be relevant and the prosecution would want to present all the proof they had.  

I was under the impression that all killings of humans were considered homicide, intent be damned. The intention of the defendant would only come into question when determining whether they are responsible for murder, manslaughter, or a legal homicide.

Is this incorrect?

You're right, and thus I missed the thrust of your prior answer. 

Thank you so much, RotE, I think the two possible fallacies you offer answer my question perfectly.

I guess you could say that. What was clouding my mind I suppose is that in the example I give of motive it's not as if we wouldn't expect the reality of a motive (except, say, for a random killing or manslaughter) it's just that we don't need to know it (if we limit ourselves to responsibility for the homicide). When I think of the moon landings too I'd expect explanations for things that moon hoaxers question, it's just that if we accept the artefactual evidence as being genuine we don't need to know the explanations, we just make the inference they're there to be found if you really want to find them. In other cases, the red herring raised might involve information that is both unfindable and irrelevant - for example, temperature when talking about measurement - but as far as the logical fallacy goes whether the information exists or doesn't, it's the needing to know it that's the relevant point.

I really shouldn't have used motive as an example because I really meant it in a very hypothetical sense - simply that motive isn't required if there is clear evidence of who is responsible, not a real life situation in a courtroom. 

As Shawn pointed out motive does make a difference in terms of classification of the crime so in that instance it is relevant and it's not as if it isn't a "good-to-know" type situation, it's always good if a motive can be identified because otherwise the question arises as to why a person with no motive would kill someone unless it was some random-type killing. However, in the purely hypothetical sense, a motive isn't needed to be known if there is clear evidence that someone was responsible for the killing of someone else.

If we move to the subject that prompted my question, argument against the moon landings by the moon hoaxers, it is now clearer in my mind the error in argument they make. They ignore the very compelling artefactual evidence in favour of abstruse argument about space science they have little knowledge of. I have little knowledge myself and that's why I stick to the artefactual evidence because it is so compelling and, as far as I'm concerned, artefactual evidence has top priority. Its actual existence (whether real or faked) cannot be argued with although interpretation of it can which is a better starting point than other kinds of evidence that, in the first place, doesn't have a clear existence that can be agreed upon.

I would never argue that logical fallacy is always deliberate - if it were I think it would be easier to argue against! Unfortunately, I believe a lot of logical fallacy is simply driven by people's inclination to believe one way or another so they pull out a logical fallacy without even realising ... and when you try to explain they've used one they simply reject what you say. Moon hoaxers indulge shamelessly in the logical fallacy, argumentum ad speculum, in their argument, "If we'd gone to the moon, we would have gone again." They just love that fallacy and simply won't have that it's a fallacious argument.