Sombart, Werner. 1894. “Zur Kritik des ökonomischen Systems von Karl Marx” [Toward a Critique of the Economic System of Karl Marx], Archiv für soziale Gesetzgebung und Statistik 7: 555–594.Wilbrandt (1919: 100) appears to have taken the same view as in Engels’ supplement that the law of value in volume 1 of Capital was to be confined only to the pre-modern world of commodity exchange, while Sombart and Schmidt struggled to see how the law of value in volume 1 was empirical at all in light of volume 3.
Lexis, W. 1895. “The Concluding Volume of Marx’s Capital,” Quarterly Journal of Economics 10 (October): 1–33.
Schmidt, Conrad. 1895. “Der dritte Band des Kapital,” Sozialpolitisches Zentralblatt 22 (25th February): 254–258.
Sorel, G. 1897. “Sur la théorie marxiste de la valeur,” Journal des Economistes (March): 222–231.
Wilbrandt, Robert. 1919. Karl Marx: Versuch einer Würdigung. B. G. Teubner, Leipzig.
Engels had a correspondence with both Sombart and Schmidt on the labour theory of value, and this is what prompted Engels to write the Supplement to volume 3.
In fact, it was in a letter that Engels wrote to Werner Sombart (1863–1941) on March 11, 1895 that we have this admission:
“When commodity exchange began, when products gradually turned into commodities, they were exchanged approximately according to their value. It was the amount of labour expended on two objects which provided the only standard for their quantitative comparison. Thus value had a direct and real existence at that time. We know that this direct realisation of value in exchange ceased and that now it no longer happens.”The importance of this letter and Engels’ supplement cannot be stressed enough for understanding and refuting the labour theory of value.
Letter, Engels to W. Sombart, from London, March 11, 1895