Failing to find any plausible contrast, we realize that the modifier 'directly' doesn't do any work here: it is meaningless. — SophistiCat

This sounds right to me, and reduces contrast theory to the principle of a Venn diagram. a word has meaning by making a distinction between what it refers to and everything else, with the distinction drawn as a line between them.

We can know what a unicorn is - a magical horse like creature with a single horn on its head - even though we know there are no unicorns. but when we want t make useful functional distinctions, between forms of seeing and such, there has to be something on each side of the line for the classification to function. To say all seeing is direct, or all seeing is indirect does not draw a line in the world of seeing at all. The distinction does not function in the world of seeing unless it divides seeing into contrasting segments:- I see directly what is in front of me, and indirectly via the rear view mirror or via a camera or other apparatus —. and then we can argue whether spectacles, rose tinted or not, are to count as an apparatus or not.