“Asylums are nothing more than gardens of human cabbages, of miserable, grotesque, repugnant human beings watered with the fertilizer of injections.”

1900s, God Does Not Exist (1904)

Source: Manhood of Humanity (1921), p. 133. Chapter: Capitalistic Era.
Context: To regard human beings as tools — as instruments — for the use of other human beings is not only unscientific but it is repugnant, stupid and short sighted. Tools are made by man but have not the autonomy of their maker — they have not man's time-binding capacity for initiation, for self-direction, and self-improvement.

Reported in Proceedings in honor of Mr. Justice Frankfurter and distinguished alumni at the meeting of the Council, Harvard Law School Association in Cambridge, April 30, 1960.
Other writings

Essays, Are Women Human? (1938)

Source: Angels & Demons

Cornell Chronicle interview (1999)
Context: We are human, and nothing is more interesting to us than humanity. The appeal of literature is that it is so thoroughly a human thing — by, for and about human beings. If you lose that focus, you obviate the source of the power and permanence of literature.

Source: Autobiography of Mark Twain, Vol. 2 (2013), p. 136

Source: We Are All Completely Beside Ourselves