What makes the abstract understanding, the modern and the ancient one, in its individual parts or categories as well as in its totality so puzzling, is the fact that in it any perceptual reality is abstracted, no trace of sense activity is contained, and that an understanding of such a nature can nevertheless serve, indeed is downright indispensable, to investigate perceptible nature, namely to investigate what has general necessity of nature, mathematically definable natural law, in the phenomena that can be experienced.
For it is indeed the use of non-empirical abstractions that gives this mode of thinking its conceptual form and the specific intellectual character by which it is essentially divorced from manual labor, but equally essentially superior to it in the scope of cognition. Despite all criticism of the >>deficiencies of abstract natural-scientific materialism, which excludes the historical process<< (MEW Vol. 23, p. 393), Marx attributes to this intellect the ability of a >>scientific analysis<<, which makes phenomena accessible to us, >>just as the apparent movement of the heavenly bodies is comprehensible only to those who know their real, but sensually imperceptible movement<< (ibid., p. 335).
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SOHN-RETHEL, Alfred, 1990. Das Geld, die bare Münze des Apriori. Berlin: Wagenbach. Kleine kulturwissenschaftliche Bibliothek, 27. ISBN 978-3-8031-5127-8.