in Thoreau’s Journal:

It requires a different intention of the eye in the same locality to see different plants…. I find that when I am looking for the former, I do not see the latter in their midst. How much more, then, it requires different intentions of the eye and of the mind to attend to different departments of knowledge ! How differently the poet and the naturalist look at objects! A man sees only what concerns him. A botanist absorbed in the pursuit of grasses does not distinguish the grandest pasture oaks.

He as it were tramples down oaks unwittingly in his walk.