I remember Eleanor Harz Jorden's explanation of this phenomenon. She actually uses different words to describe Japanese grammar, since using the English words like noun and adjectives is misleading.
But, basically, according to her any verb becomes an adjective in the negative. Anything you can do to an adjective you can do to a negative verb, even turn it into an adverb. So, indeed, here the verb 見える takes on a form that modifies verbs. Conceptually, first it becomes the adjective 見えない and then the adverb 見えなく.
The only exception is that it usually doesn't make any sense to negate a negative verb, so you wouldn't say 見えなくない. "It's not not visible." But you should be able to do anything else. 見えなさそう and so on are fine, conjugated just like adjectives. I don't know if Tae Kim talks about this anywhere.
Yeah, it's basically an adverb
I remember Eleanor Harz Jorden's explanation of this phenomenon. She actually uses different words to describe Japanese grammar, since using the English words like noun and adjectives is misleading.
But, basically, according to her any verb becomes an adjective in the negative. Anything you can do to an adjective you can do to a negative verb, even turn it into an adverb. So, indeed, here the verb 見える takes on a form that modifies verbs. Conceptually, first it becomes the adjective 見えない and then the adverb 見えなく.
The only exception is that it usually doesn't make any sense to negate a negative verb, so you wouldn't say 見えなくない. "It's not not visible." But you should be able to do anything else. 見えなさそう and so on are fine, conjugated just like adjectives. I don't know if Tae Kim talks about this anywhere.