The slave frees himself when, ...

The slave frees himself when, of all the relations of private property, he abolishes only the relation of slavery and thereby becomes a proletarian; the proletarian can free himself only by abolishing private property in general.
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 Friedrich Engels

Quotes from the same author

Thus at every step we are reminded that we by no means rule over nature like a conqueror over a foreign people, like someone standing outside nature - but that we, with flesh, blood and brain, belong to nature, and exist in its midst, and that all our mastery of it consists in the fact that we have the advantage over all other creatures of being able to learn its laws and apply them correctly.
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 Friedrich Engels
Competition permits the capitalist to deduct from the price of labour power that which the family earns from its own little garden or field; the workers are compelled to accept any piece wages offered to them, because otherwise they would get nothing at all, and they could not live from the products of their small-scale agriculture alone, and because, on the other hand, it is just this agriculture and landownership which chains them to the spot and prevents them from looking around for other employment.
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 Friedrich Engels
Ireland still remains the Holy Isle whose aspirations must on no account be mixed with the profane class-struggles of the rest of the sinful world ... the Irish peasant must not on any account know that the Socialist workers are his sole allies in Europe.
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 Friedrich Engels
And what is impossible to science?
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 Friedrich Engels
The British Labour movement is today, and for many years has been, working in a narrow circle of strikes that are looked upon, not as an expedient, and not as a means of propaganda, but as an ultimate aim.
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 Friedrich Engels