U.S.writer, note esp. for Walden, or Life in the Woods (1854), an account of his experiment in living in solitude. A powerful social critic, his essay Civil Disobedience (1849) influenced such dissenters as Gandhi.

various extracts from 'Walden':
It may be vain to ask why the imagination will not be reconciled to flesh and fat. I am satisfied that it is not. Is it not a reproach that man is a carnivorous animal? True, he can and does live, in a great measure, by preying on other animals; but this is a miserable way - as anyone who will go to snaring rabbits, or slaughtering lambs, may learn - and he will be regarded as a benefactor of his race who shall teach man to confine himself to a more innocent and wholesome diet. Whatever my own practice may be, I have no doubt that it is a part of the destiny of the human race, in its gradual improvement, to leave off eating animals, as surely as the savage tribes have left off eating each other when they came in contact with the more civilised.
The practical objection to animal food in my case was its uncleanness; and, besides, when I had caught, and cleaned, and cooked, and eaten my fish, they seemed not to have fed me essentially. it was insignificant and unnecessary, and cost more than it came to. A little bread or a few potatoes would have done as well, with less trouble and filth. Like many of my contemporaries, I had rarely for many years used animal food, or tea, or coffee, etc.: not so much because of any ill effects which I had traced to them, as because they were not agreeeable to my imagination. The repugnance to animal food is not the effect of experience, but is an instinct. It appears more beautiful to live low and fare hard in many respects; and though I never did so, I went far enough to please my imagination. I believe that every man who has ever been earnest to preserve his higher or poetic faculties in the best condition has been particularly inclined to abstain from animal food, and from much food of any kind.
No humane being, past the thoughtless age of boyhood, will wantonly murder any creature which holds its life by the same tenure that he does.
I once had a sparrow alight on my shoulder for a moment while I was hoeing in a village garden, and I felt that I was more distinguished by that circumstance than I should have been by any epaulet I could have worn.
I have found repeatedly, of late years, that I cannot fish without falling a little in self-respeet. I have tried it again and again. I have skill at it,and, like many of my fellows, a certain instinct for it, which revives from time to time, but always when I have done I feel it would have been better if I had not fished. I think that I do not mistake. It is a faint intimation, yet so are the first streaks of morning.
Other quotes:
I saw a muskrat come out of a hole in the ice ... While I am looking at him, I am thinking what he is thinking of me. He is a different sort of man, that's all. - Journal
The squirrel that you kill in jest, dies in earnest. - Familiar Letters
Our science, so called, is always more barren and mixed with error than our sympathies. - Familiar Letters
I do not consider the other animals brutes in the common sense. I am attracted toward them undoubtedly because I never heard any nonsense from them. I have not convicted them of folly or vanity or pomposity or stupidity in dealing with me. Their vices, at any rate, do not interfere with me. - Familiar Letters
There are a thousand hacking at the branches of evil to one who is striking at the root. - unknown origin
Spring - An experience in immortality. - unknown origin
All good things are wild, and free. - unknown origin
If I repent of anything it is likely to be my good behavior. - unknown origin
If...the machine of government...is of such a nature that it requires you to be the agent of injustice to another, then, I say, break the law. - unknown origin
I have just been through the process of killing the cistudo for the sake of science; but I cannot excuse myself for this murder, and see that such actions are inconsistent with the poetic perception, however they may serve science, and will affect the quality of my observations. I pray that I may walk more innocently and serenely through nature. No reasoning whatever reconciles me to this act. It affects my day injuriously. I have lost some self-respect. I have a murderer's experience to a degree. - The Heart of Thoreau's Journals