Albrecht Dürer, study of hands, chalk drawing on blue paper, n.d.

The question whether objective [gegenständliche] truth can be attributed to human thinking is not a question of theory but is a practical question. In practice man must prove the truth, that is, the reality and power, the this-sidedness [Diesseitigkeit] of his thinking. The dispute over the reality or non-reality of thinking which is isolated from practice is a purely scholastic question.

Second thesis on Feuerbach, 1845