If authority just counts as evidence

I have an intuition that insofar as a person is fallible, what she says can only be authoritative over what I believe when what she says counts as strong or overriding evidence. Maybe what she says has sway over what I ought to do regardless (even in the case of a mistake) but she cannot influence what I ought to think unless she's an epistemic expert.

Here's a scenario that sheds some light on why my intuition might be off:

I'm on the roof of a burning building and need to get to the building across the alley. The only way to cross is by walking a narrow plank. Firefighter Jim says to me, "Go ahead, it's safe to cross." If there is a net below that I don't know about (because I refuse to look down or something), then it seems obvious that Jim is right, regardless of my skill in balancing. If there is no net, the probability that Jim is right decreases significantly. Is he an authority in both cases? The issue arises in the second case: the reason I have to cross is explained by the circumstances- the fire quickly rising in the building, threatening my life if I stay put. I don't need Jim's authority to explain why I ought to cross. But ought I to believe it's safe to cross? All the stories I can think of would just explain why what Jim says is authoritative only for pragmatic reasons. If I believe him and the statement is false, I still may have a higher chance of making it over because I walk across more confidently. But maybe Jim knows this; then it looks like there's a real reason I ought to believe him and do what he says, not just do what he says.

Aquinas against Platonism

Dr. Alfonso Gomez-Lobo today pointed out to me the problem Aquinas has with Platonism . The argument runs:

1. If Plato's divided line is true, then all principles proceed from one principle- the form of the good.

2. If two or more principles proceed from one principle, the multiple principles are ultimately commensurable.

3. The law of non-contradiction is a logical principle.

4. The law of inertia is a logical principle.

5. The law of non-contradiction is incommensurable with the law of inertia.

6. There exist two principles which are not commensurable (3, 4, 5).

7. It is not the case that for all principles, they proceed from one principle (2, 6).

8. Plato's divided line is false (1, 7).

Knowledge as Ability

I had a song stuck in my head yesterday morning, and as often happens with me, I caught myself humming along to the music in my mind. Of course, I can’t hum the whole song all at once (I’ve heard that some professional singers can sing two notes at once, but I am far from mastering any technique like that). But I would say that I know the song, and that, given an instrument with the capability to play multiple notes at once, I could replicate the song. I would have to have certain skills on this instrument. If I lacked those skills, would I still know the song? I certainly couldn’t prove it. And there are more skills that I would need to learn to play an instrument in the first place. How could I acquire any of these abilities without primitive ones?

If knowledge is, as the Sellarsians say, the ability to participate in the game of giving and asking for reasons, I don’t have knowledge. The regress of requirements for knowledge appears to me to be a vicious one. Unless we have innate abilities, Sellarsian knowledge is unattainable.