John Stuart Mill

That a thing is unnatural, in any precise meaning which can be attached to the word, is no argument for its being blamable; since the most criminal actions are to a being like man not more unnatural than most of the virtues.
On Nature
In early times, the great majority of the male sex were slaves, as well as the whole of the female. And many ages elapsed, some of them ages of high cultivation, before any thinker was bold enough to question the rightfulness, and the absolute social necessity, either of the one slavery or of the other.
The Subjection of Women
That the principle which regulates the existing social relations between the two sexes — the legal subordination of one sex to the other — is wrong itself, and now one of the chief hindrances to human improvement; and that it ought to be replaced by a principle of perfect equality, admitting no power or privilege on the one side, nor disability on the other.
The Subjection of Women
Let not any one pacify his conscience by the delusion that he can do no harm if he takes no part, and forms no opinion. Bad men need nothing more to compass their ends, than that good men should look on and do nothing.
Inaugural Address Delivered to the University of St. Andrews
War is an ugly thing, but not the ugliest of things: the decayed and degraded state of moral and patriotic feeling which thinks that nothing is worth a war, is much worse.
The Contest in America
It is better to be a human dissatisfied than a pig satisfied; better to be Socrates dissatisfied than a fool satisfied.
If all mankind minus one, were of one opinion, and only one person were of the contrary opinion, mankind would be no more justified in silencing that one person, than he, if he had the power, would be justified in silencing mankind.
On Liberty
The only purpose for which power can be rightfully exercised over any member of a civilized community, against his will, is to prevent harm to others.
On Liberty
As there were black swans, though civilized people had existed for three thousand years on the earth without meeting with them...The uniform experience, therefore, of the inhabitants of the known world, agreeing in a common result, without one known instance of deviation from that result, is not always sufficient to establish a general conclusion.
A System of Logic