…during this long period from Descartes to Hegel and from Hobbes to Feuerbach, these philosophers were by no means impelled, as they thought they were, solely by the force of pure reason. On the contrary, what really pushed them forward most was the powerful and ever more rapidly onrushing progress of natural science and industry. Among the materialists this was plain on the surface, but the idealist systems also filled themselves more and more with a materialist content and attempted pantheistically to reconcile the antithesis between mind and matter. Thus, ultimately, the Hegelian system represents merely a materialism idealistically turned upside down in method and content.
Friedrich Engels, Ludwig Feuerbach and the End of Classical German Philosophy, 1886
Full text at Marxists Internet Archive