Marx was already aware of this theoretical problem when he wrote "The Poverty of Philosophy" (1847). It gets a mention again in the "Grundrisse" (1858). At the end of chapter 1 of his "A Contribution to the Critique of Political Economy" (1859), he referred to it, and announced his intention to solve it. In "Theories of Surplus Value" (1862–1863), he discusses the problem very clearly. His first attempt at a solution occurs in a letter to Engels, dated 2 August 1862. In "Capital, Volume I" (1867) he noted that "many intermediate terms" were still needed in his progressing narrative, to arrive at the answer. Engels suggested, that Marx had indeed solved the problem in the posthumously published "Capital, Volume III", but critics alleged Marx never delivered a credible or definitive solution.