 Frege was mostly interested in formalizing arithmetic, and so considered the logical forms of a relative minority of natural language sentences in a deliberately spare notation – he didn’t take on the likes of (12)-(13). But work on the logical (or syntactic) structure of the full range of sentences of natural language has blossomed since then, initially in the work of Bertrand Russell (1905), in his famous theory of definite descriptions (see Descriptions), which he (1912) combined with his views about the knowledge by “acquaintance” with sense-data and universals into a striking “fundamental principle in the analysis of propositions containing descriptions”: