“In whatever state of knowledge we may conceive man to be placed, his progress towards a yet higher state need never fear a check, but must continue till the last existence of society.”

The History of America, Vol. I (1777), Book IV, pp. 281–282

Book IX, Ch. 3
Progress and Poverty (1879)

Astronomical Observations relating to the Construction of the Heavens... (1811), p. 318

The Snow Leopard (1978)
Context: The progress of the sciences toward theories of fundamental unity, cosmic symmetry (as in the unified field theory) — how do such theories differ, in the end, from that unity which Plato called “unspeakable” and “indiscribable,” the holistic knowledge shared by so many peoples of the earth, Christians included, before the advent of the industrial revolution made new barbarians of the peoples of the West? In the United States, before spiritualist foolishness at the end of the last century confused mysticism with “the occult” and tarnished both, William James wrote a master work of metaphysics; Emerson spoke of “the wise silence, the universal beauty, to which every part and particle is equally related, the eternal One . . .”; Melville referred to “that profound silence, that only voice of God”; Walt Whitman celebrated the most ancient secret, that no God could be found “more divine than yourself.” And then, almost everywhere, a clear and subtle illumination that lent magnificence to life and peace to death was overwhelmed in the hard glare of technology. Yet that light is always present, like the stars of noon. Man must perceive it if he is to transcend his fear of meaningless, for no amount of “progress” can take its place. We have outsmarted ourselves, like greedy monkeys, and now we are full of dread.

Source: The Theory of Political Economy (1871), Chapter VII, Theory of Capital, p. 191.

Final lines
Civil Disobedience (1849)
Context: Is a democracy, such as we know it, the last improvement possible in government? Is it not possible to take a step further towards recognizing and organizing the rights of man? There will never be a really free and enlightened State until the State comes to recognize the individual as a higher and independent power, from which all its own power and authority are derived, and treats him accordingly. I please myself with imagining a State at least which can afford to be just to all men, and to treat the individual with respect as a neighbor; which even would not think it inconsistent with its own repose if a few were to live aloof from it, not meddling with it, nor embraced by it, who fulfilled all the duties of neighbors and fellow-men. A State which bore this kind of fruit, and suffered it to drop off as fast as it ripened, would prepare the way for a still more perfect and glorious State, which also I have imagined, but not yet anywhere seen.

Pt. I, Ch. 2 : The Evanescence of Evil, concluding paragraph
Social Statics (1851)
Context: Man needed one moral constitution to fit him for his original state; he needs another to fit him for his present state; and he has been, is, and will long continue to be, in process of adaptation. And the belief in human perfectibility merely amounts to the belief that, in virtue of this process, man will eventually become completely suited to his mode of life.
Progress, therefore, is not an accident, but a necessity. Instead of civilization being artificial, it is part of nature; all of a piece with the development of the embryo or the unfolding of a flower. The modifications mankind have undergone, and are still undergoing, result from a law underlying the whole organic creation; and provided the human race continues, and the constitution of things remains the same, those modifications must end in completeness.

Letter to a relative, (1861).
Context: I think I have fairly heard and fairly weighed the evidence on both sides, and I remain an utter disbeliever in almost all that you consider the most sacred truths [... ] I can see much to admire in all religions [... ] But whether there be a God and whatever be His nature; whether we have an immortal soul or not, or whatever may be our state after death, I can have no fear of having to suffer for the study of nature and the search for truth.

The Rights of Free Men: An Essential Guide to Civil Liberties (1984).