Something meant to live in Air

Do trees exist?

entry for the nineteenth day of the fifth month in the year the albatross came the the south-western halls

Many things are unknown. Once — it was about six or seven months ago — I saw a bright yellow speck floating on a gentle Tide beneath the Fourth Western Hall. Not understanding what it could be, I waded out into the Waters and caught it. It was a leaf, very beautiful, with two sides curving to a point at each end. Of course it is possible that it was part of a type of sea vegetation that I have never seen, but I am doubtful. The texture seemed wrong. Its surface repelled Water, like something meant to live in Air.

— from Piranesi, by Susanna Clarke.

Here was a clue that he knew about the trees. But it is a relearning. Knowledge through observation through the senses. What really exists? How can I understand it?