Striking similarities can be observed between Grice's (1967) conversational maxims and Tauli's (1968) principles of language planning: In order to function well, a language must be such that it makes a well behaved conversation possible. Nevertheless, many ethnic languages as well as constructed interlanguages possess features which are incompatible with these principles. The paper contains an analysis of such cases: The compulsory expression of number and gender, which is in conflict with the principle of facultative precision; ambiguity and lack of distinctiveness in the names of the Latin letters; excessive length of certain morphemes; and restricted freedom to place sentence constituents in a pragmatically adequate order. The compulsory nature of certain distinctions, which cannot be understood on functional grounds, is ascribed to an excessive activity of "universal grammar", which is seen as one of the instincts of human beings. It is further shown that some constructed interlanguages (Volapük and Interlingua) clash with some of Greenberg's (1963) universals concerning the order of meaningful elements. While Esperanto is free from that type of deficiency, it is in conflict with the principle of facultative precision in the same way as most ethnic languages of Europe, as distinct from those of East Asia.