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.