Question

".....
Finds tongues in trees, books in the running brooks,
Sermons in stones, and good in every thing."
-Duke Senior in As You Like It, Act 2 Scene 1

What precisely did he mean by that? That he could learn a lot from nature?

Answer 1

In As You Like It, Duke Senior has been forced to live a life of exile in the Forest of Arden, being usurped by his own younger brother. He comes to the forest, and finds a life better than the one at the court, because here everything seems loyal to him. Though the harsh conditions make life a little difficult, he is still optimistic. He finds "tounges in trees", which means that the trees seem to speak to him. "Books in the running brooks" implies that he can stare into the stream and learn about nature just like we stare into a book and acquire knowledge by reading it."Sermons in stones" similarly imply that stones seem to give speeches(semons) to him, when he looks at it.
The paragraph is meant to show Duke Senior's optimism.

Answer 2

Ah. Danyavaad! :P