A truly good book is something as wildly natural and primitive, mysterious and marvelous, ambrosial and fertile, as a fungus or a lichen.—Journal, 16 November 1850
As it is important to consider Nature from the point of view of science remembering nomenclature and system of men, and so, if possible, go a step further in that direction, so it is equally important often to ignore or forget all that men presume they know, and take an original and unprejudiced view of Nature, letting her make what impression she will on you, as the first men, and all children and natural men still do. — Journal, 28 February 1860—Journal, 28 February 1860
But cowardice is unscientific; for there cannot be a science of ignorance. There may be a science of bravery, for that advances; but a retreat is rarely well conducted; if it is, then is it an orderly advance in the face of circumstances. — "Natural History of Massachusetts"—"Natural History of Massachusetts"
I have been making pencils all day, and then at evening walked to see an old schoolmate who is going to help make Welland Canal navigable for ships round Niagara. He cannot see any such motives and modes of living as I; professes not to look beyond securing certain "creature comforts". And so we go silently different ways . . . — Journal, 17 March 1842—Journal, 17 March 1842