It would be instructive to examine the references which there are in Marx's _Das Kapital_ to unfinished analyses, extraneous to his special method; for in this dependence on analysis the researches of pure economics have their origin. What is, for instance, _abstract human labour_ (_abstrakt menschliche Arbeit_), a concept which Marx uses like a postulate? By what _method_ is that reduction of _complex_ to _simple_ labour accomplished, to which he refers as to an obvious and ordinary matter? And if, in Marx's hypothesis, _commodities_ appear as _congealed labour_, or _crystalised labour_, why by another hypothesis, should not all economic goods and not only commodities, appear as _congealed methods of satisfying needs_ or as _crystalised needs_? I read at one point in _Das Kapital_: 'Things which in themselves are not commodities, _e.g._ knowledge, honour, etc., may be sold by their owners; and thus, by means of their price, acquire the form of commodities. A thing may formally have a price without having a value. The expression of the price here becomes _imaginary_ like certain quantities in mathematics.'[33] Here is yet another difficulty, indicated but not overcome. Where are these _formal_ or _imaginary_ prices to be found? And what are they? By what laws are they governed? Or are they perhaps like the Greek words in Latin prosody, which according to the school rule, _per Ausoniae fines sine lege vagantur_?--Questions of this kind are answered by the researches of pure economics. The philosopher Lange also, who rejected Marx's law of value, which he regarded as an _extravagant production_, _a child of sorrow_, thinking it unsuitable--and in this he was justified, as a general law of value, arrived at the solutions which have since been given of the latter, a long time before the researches of the purists came into blossom. 'Some years ago,' he wrote in his book on labour problems, 'I too worked at a new theory of value, _which should be of such a character as to show the most extreme cases of variation in value as special cases of the same formula_.' And, whilst adding that he had not completed it, he intimated that the course which he attempted was the same as that hastily glanced at by Jevons in his _Theory of political economy_, published in 1871.[34]