“It is usually assumed that, in speaking, in the 1844 Manuscripts, of man’s “being reduced to the level of the animals,” and of man’s alienation from his “species-being” under the conditions of capitalist production, Marx is thinking in terms of an abstract conception of “man” as being alienated from his biological characteristics as a species. So, it is presumed, at this initial stage in the evolution of his thought, Marx believed that man is essentially a creative being whose “natural” propensities are denied by the restrictive character of capitalism. Actually, Marx holds, on the contrary, that the enormous productive power of capitalism generates possibilities for the future development of man which could not have been possible under prior forms of productive system. The organization of social relationships within which capitalist production is carried on in fact leads to the failure to realize these historically generated possibilities. The character of alienated labor does not express a tension between “man in nature” (non-alienated) and “man in society” (alienated), but between the potential generated by a specific form of society—capitalism—and the frustrated realization of that potential. What separates man from the animals is not the mere existence of biological differences between mankind and other species, but the cultural achievements of men, which are the outcome of a very long process of social development.”

Part 2, Section 12
A Treatise of Human Nature (1739-40), Book 2: Of the passions

Credo (1965)

Source: Giovanni Gentile: Philosopher of Fascism, (2001), p. 55

1950s, The Russell-Einstein Manifesto (1955)

Source: The Limits of State Action (1792), Ch. 3

On his motivations to write Lord of the Flies, from his essay "Fable", p. 85
The Hot Gates (1965)
Context: The overall intention may be stated simply enough. Before the Second World War I believed in the perfectibility of social man; that a correct structure of society would produce goodwill; and that therefore you could remove all social ills by a reorganisation of society..... but after the war I did not because I was unable to. I had discovered what one man could do to another... I must say that anyone who moved through those years without understanding that man produces evil as a bee produces honey, must have been blind or wrong in the head... I am thinking of the vileness beyond all words that went on, year after year, in the totalitarian states. It is bad enough to say that so many Jews were exterminated in this way and that, so many people liquidated — lovely, elegant word — but there were things done during that period from which I still have to avert my mind less I should be physically sick. They were not done by the headhunters of New Guinea or by some primitive tribe in the Amazon. They were done, skillfully, coldly, by educated men, doctors, lawyers, by men with a tradition of civilization behind them, to beings of their own kind.
My own conviction grew that what had happened was that men were putting the cart before the horse. They were looking at the system rather than the people. It seemed to me that man’s capacity for greed, his innate cruelty and selfishness, was being hidden behind a kind of pair of political pants. I believed then, that man was sick — not exceptional man, but average man. I believed that the condition of man was to be a morally diseased creation and that the best job I could do at the time was to trace the connection between his diseased nature and the international mess he gets himself into. To many of you, this will seem trite, obvious, and familiar in theological terms. Man is a fallen being. He is gripped by original sin. His nature is sinful and his state is perilous. I accept the theology and admit the triteness; but what is trite is true; and a truism can become more than a truism when it is a belief passionately held....
I can say in America what I should not like to say at home; which is that I condemn and detest my country's faults precisely because I am so proud of her many virtues. One of our faults is to believe that evil is somewhere else and inherent in another nation. My book was to say you think that now the war is over and an evil thing destroyed, you are safe because you are naturally kind and decent. But I know why the thing rose in Germany. I know it could it could happen in any country. It could happen here.

Source: The Limits of Evolution, and Other Essays, Illustrating the Metaphysical Theory of Personal Ideaalism (1905), The Limits of Evolution, p.47

As A Man Thinketh (1902)
Source: As a Man Thinketh

Source: A History of Economic Thought (1939), Chapter VI, Marx, p. 295