What kind of fallacy or false logic is this?

Question

I got into an argument with a friend on MMA fighters and said that any fighter that gets caught PEDs should probably have an asterisk by his record. His defence was that could I say for sure that other fighters aren't using PEDs just because they haven't been caught. All that means is that they haven't been caught.

This kept going in circles and I kept saying that you can only go off what evidence you have but he kept repeating and then we just dropped it. I'm pretty sure his argument has to be logically flawed and was hoping for a better explanation of why.

Thank you

Sorry I didn't respond sooner. I thought some more about the conversation I had and I didn't structure my post properly. So when I said that the MMA fighter should have an asterisk next to his record, he argued that they shouldn't and those wins should count because you can't prove that others weren't cheating. What I'm having a hard time with is explaining that you can't make decisions about things you don't know. If the other fighters are using PEDs, and we have evidence then they should get punished too, but until then you shouldn't punish nobody because you can't prove others aren't using.

At this point I'm not particularly looking for a fallacy but I just want to know if his way is a valid approach. That you can't punish fighter X because fighter y is also cheating but just got lucky and wasn't caught.

Answer 1

Some general thoughts:

First:

The is/ought dichotomy suggests that there is no valid way to move from what is, to what ought to be.

You are saying: If we can prove X, then we should do Y. You're friend is saying: Let's not do Y because we can't prove R.

You've run into the is/ought dichotomy, so unless you can frame the context within an ethical system that you and your friend both agree on and will take its principles as axioms, then you're not going to get far.

What you can do, though, is try to break the discussion down into general principles:

Premise 1 (P1): Cheating is unfair.
P2: Cheaters should be punished.
P3: For all MMA fighters, if they are caught cheating, then we should do Y to them.
P4: Using PEDs is cheating.

P1 and P2 should be easy to get consent on. P3 is going to be a hard sell. You don't have a way to logically PROVE that Y is the appropriate punishment, and hopefully this forces you to recognize your own assumptions/biases (not that you should abandon them, but you should be aware of them).

Secondly:

I suspect you were remembering the fallacy called an appeal to ignorance where someone says: Because you cannot prove ~X, then X is true. The classical example would be: There is proof that ghosts are not real, therefore ghosts are real.

You're friend is saying: You cannot prove Fighter 2 isn't using Drugs.
That LOOKS like: You can't prove ~D,

but your friend is NOT saying: You can't prove Fighter 2 isn't using drugs, therefore he is using drugs. So your friend is not committing this fallacy, strictly speaking.

Thirdly:

Logic only applies to statements (sentences) expressed in the indicative mood. It sounds, based on your post, that you and your friend were speaking in the subjunctive mood. It doesn't mean that you couldn't translate your discussion into an argument, but the fact you are using the wrong mood is an indicator that logic may not the right tool for the job.

References:
Is-ought dichotomy
Subjunctive Mood