But it is by no means clear that ‘the whole difficulty’ is thereby ‘overcome’. Russell makes a clear identification of the principle he cites (a version of CP) as the source of error, but this does not in the least make it clear what is to take its  place.[2]  In his Grundgesetze (see e.g., Frege 1903: §146–147) Frege recognises that his (in)famous Law V is based on a conversion principle which allows us to assume that for any concept (function), there is an object which contains precisely those things which fall under that concept (or for which the function returns the value ‘True’). Law V is then the principle which says that two such extension objects a, b stemming from two concepts F, G are the same if, and only if, F and G are extensionally equivalent. Frege clearly considers the ‘conversion’ of concepts to extensions as fundamental; he also regards it as widely used in mathematics (even if only implicitly), and thus that he is not ‘doing anything new’ by using such a principle of conversion and the attendant ‘basic law of logic’, Law V. (The CP follows immediately from Law V.) Frege was made aware by Russell (1902) that his Law V is contradictory, since Russell's paradox flows easily from it. In the Appendix to Grundgesetze (Frege 1903), Frege says this: