The chief defect of all previous materialism (including Feuerbach's) is that the concrete thing, the real, the perceptible is considered to an be object or [datum of] perception only and not to be perceptible human activity, or praxis, i.e. it is not considered subjectively. That is why this active aspect or side has been developed abstractly, in opposition to materialism, by idealism, which of course ignores real sensible activity as such. Feuerbach wants sensible objects that are really distinct from thought objects; but he does not grasp the fact that human activity itself is objective activity. Hence in The Essence of Christianity he considers only theoretic activity to be truly human, while practical activity is conceived only in its grubby Jewish form of expression and is equated with that. Thus he does not grasp the significance of ‘revolutionary’ activity, of ‘practical-critical’ activity.

The question of whether human thinking attains objective truth is not a question of theory but a practical question. It is in practice that man must prove the truth, the actuality and power, the subjective aspect and validity [Diesseitigkeit] of his thinking. Argument about the actuality or non-actuality of thinking, where thinking is taken in isolation from practice, is a purely scholastic question.

The materialist doctrine regarding changes [in man] brought about by circumstances and by education forgets that circumstances are changed by men and that the educator must himself be educated.