…there is yet a set of different philosophers — those who question the possibility of any cognition, or at least of an exhaustive cognition, of the world. To them, among the more modern ones, belong Hume and Kant, and they played a very important role in philosophical development. What is decisive in the refutation of this view has already been said by Hegel, in so far as this was possible from an idealist standpoint. The materialistic additions made by Feuerbach are more ingenious than profound. The most telling refutation of this as of all other philosophical crotchets is practice — namely, experiment and industry. If we are able to prove the correctness of our conception of a natural process by making it ourselves, bringing it into being out of its conditions and making it serve our own purposes into the bargain, then there is an end to the Kantian ungraspable “thing-in-itself”. The chemical substances produced in the bodies of plants and animals remained just such “things-in-themselves” until organic chemistry began to produce them one after another, whereupon the “thing-in-itself” became a thing for us — as, for instance, alizarin, the colouring matter of the madder, which we no longer trouble to grow in the madder roots in the field, but produce much more cheaply and simply from coal tar. For 300 years, the Copernican solar system was a hypothesis with 100, 1,000, 10,000 to 1 chances in its favour, but still always a hypothesis. But then Leverrier, by means of the data provided by this system, not only deduced the necessity of the existence of an unknown planet, but also calculated the position in the heavens which this planet must necessarily occupy, and when [Johann] Galle really found this planet [Neptune, discovered 1846, at Berlin Observatory], the Copernican system was proved. If, nevertheless, the neo-Kantians are attempting to resurrect the Kantian conception in Germany, and the agnostics that of Hume in England (where in fact it never became extinct), this is, in view of their theoretical and practical refutation accomplished long ago, scientifically a regression and practically merely a shamefaced way of surreptitiously accepting materialism, while denying it before the world.
Frederick Engels, Ludwig Feuerbach and the End of Classical German Philosophy, 1886
Full text at Marxists Internet Archive