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Feuerbach resolves the religious essence into the human essence. But the human essence is no abstraction inherent in each single individual. In reality it is the ensemble of the social relations. Feuerbach, who does not enter upon a criticism of this real essence, is consequently compelled [t]o abstract from the historical process and to fix the religious sentiment as something by itself and to presuppose an abstract – isolated – human individual....
Feuerbach, consequently, does not see that the “religious sentiment” is itself a social product, and that the abstract individual whom he analyses belongs to a particular form of society.
All social life is essentially practical. All mysteries which lead theory to mysticism find their rational solution in human practice and in the comprehension of this practice. ("Theses on Feuerbach," VI-VIII)
It is only the abstract individual who needs or imagines a mystical experience (of absolute passivity and dependence, no less) with the all, the universe, the infinite - only the individual whose life of relations and work has lost all sense of human meaning. (Marx isn't interested in mystics of the past.) A critique no less compelling and unsettling today than it was in 1843. (I also gave students the section from Kapital where Marx famously describes the fetishism of commodities, so they knew what this abstraction referred to - the displacement of use value by exchange value, where everything's interchangeable with everything else, value and labor and time are abstracted units... I explained it better here.)
(The text at the top is Marx's autograph of the last and most famous of the "Theses on Feuerbach": Die Philosophen haben die Welt nur verschieden interpretiert, es kömmt darauf an, sie zu verändern.)