The classical logical fallacy of the ad hominem attack is attacking the person, not the argument.
This is one of the more complicated logical fallacies, because it’s valid sometimes. It’s valid because humans are not omniscient.
Suppose an astronaut and a skeptic are arguing. The astronaut says the Earth is round. The skeptic says that’s wrong. The astronaut says it isn’t because the astronaut has been in space and looked down, seeing the spherical shape. The skeptic calls the astronaut a liar.
The thing is, the astronaut’s argument is fundamentally an appeal to authority but may be valid. You can do trig on shadows or look at Moon pictures to ascertain the curvature of the Earth, but the validity of an argument is not impinged by other valid arguments, especially if they support the first argument. Regardless of other viable arguments, ‘I went to space and looked at the Earth, and it’s a ball’ is a powerful argument. That’s in effect the fundamental principle of science. One observed something and based conclusions on observations.
But the skeptic hasn’t been to space. Not too many people go to space yet. That’s not going to change soon. So the argument ‘I went to space and looked at the Earth’ is limited, and more trickily, it’s based on a hidden premise. ‘I’m not lying.’ The train of logic and inherent premise is the same for things like pictures or other data. Perhaps ‘lying’ isn’t the applicable word for deception, but deception exists. Bad observation exists. See Pons and Fleischmann.
So here we are. The skeptic cannot duplicate the astronaut’s round-Earth observation. Let us not quibble about fictional possibilities. For most people within the next ten years, going into space and looking at the curvature of the Earth isn’t a plausible option. But equally clearly someone merely making an observation, or claiming to make an observation, shouldn’t be taken as unassailable, even if the other party can’t necessarily duplicate it. Again, look at Pons and Fleischmann.
Pure logic constrains itself to absolutes, but often fails when applied real world phenomena. Look at Aristotle’s natural philosophy which is, in fact, wrong. Good logic, just wrong. And if you can’t interrogate the data or duplicate the experiment, then how do you evaluate the argument?
The skeptic must look at avenues of attack.
Can the data be forged? Maybe. An equally valid question is can the skeptic figure out every way in which the data could be forged?
But suppose that isn’t available. The astronaut doesn’t have pictures to look at. The astronaut just states something: I went to space and looked, and it’s round.
There’s really no avenue of attack other than attacking the speaker because the speaker hasn’t made anything available other than themselves.
Under the dangerous play rule, this is also an invalid assertion. If one posits something in such a fashion that the only path of interrogation is itself a logical fallacy, the posited assertion is itself a fallacy.
But people can’t do everything. I’m not going into space this week, and I have to be able to evaluate an argument about it. What’s more, I’m not omniscient, and therefore can’t base my decisions on knowing everything. Can I recognize every photoshop 100% of the time? Do I know enough virology to successfully attack a scientific paper with absolute certainty of catching all errors? No.
Recall that throughout most of human history, science was wrong. Kepler postulated heliocentrism in the early 1600s. Ancient Egyptians were doing astronomy thousands of years previously. Stonehenge matches up with astronomical observation to a high degree of accuracy and it was made millennia ago too. Same with American pyramids and Asian ones.
Heck, look directly at Kepler himself. His first book of cosmology Mysterium Cosmographicum just wasn’t right either.
Same thing happened with germ theory, successor to miasmas, and so on. How are we to know we’ve got it right now?
We don’t. See, that’s the difference between philosophy and science. Philosophy seeks truth. Science seeks accuracy. We don’t know if we’ve got it right, if we have the truth of cosmology, and we’re probably making some bad assumptions, but we do know how accurate our mapping of the world is, and we can measure that.
Tangent aside, how does the skeptic evaluate the astronaut’s argument?
Scientifically, the only thing one can do is use that assertion to make other tests. Travel around and look at the Moon. Observation of the apparent rotation of the Moon supports round-Earth theory is supported; observation contradicts flat-Earth theory. In effect, round-Earth theory is more accurate.
But what if you can’t do that right now either? Because you can’t travel. Due to, say, Covid.
Well, that’s a kicker.
The skeptic is back to evaluating the astronaut’s argument in a vacuum, and then the paths are ad hominem attack or blind acceptance.
All puns intentional.
Coincidentally, these are some of the trains of thought that make me a scientist, not a philosopher. I think seeking truth above all is an impossible mission. Seeking accuracy is fairly straightforward, though it would be unsatisfying to Lewis. A good counterargument is that working for an impossible mission is still a way of getting good progress if not perfection, but isn’t aiming for good progress not perfection effectively the same thing as seeking accuracy instead of truth?
I’m going to continue this tomorrow.