More recently, he has described "universal grammar" as having a crystalline form, comparing it to a snowflake. In other words, a formalism (i.e. a syntactic model) is used to reveal hidden patterns or symmetries underlying human language. This practice became opposed by American "functionalism" which argues that language is not crystallized but dynamic and ever-changing. This type of functionalism includes various frameworks which are inspired by memetics and linked with the cognitive linguistics of George Lakoff and his associates. Like Wundt, Lakoff also proposes a psychologism for mathematics. Some frameworks advocating mathematical formalism do however exist today. Categorial grammar is a type of generative grammar which was developed by mathematicians and logicians including by Kazimierz Ajdukiewicz, Yehoshua Bar-Hillel, and Joachim Lambek. Their method includes a separate model for syntax and semantics. Thus, even categorial grammar includes a meaningful component. It is however not psychologistic because it does not claim that syntactic structures stem from human psychology; nor is it logicistic because, unlike Husserl, it does not consider structures of natural language as being logical. Furthermore, unlike structuralism, their approach adheres to a mathematical rather than a semiotic view of language. Such a framework, then, is purely descriptivist and atheoretical—that is, it does not aim to explain why languages are the way they are—or only theoretical as pertains to the concept of the word 'theory' in mathematics, especially model theory. Ideas A central assumption of linguistic formalism, and of generative linguistics in particular, is called the autonomy of syntax, according to which syntactic structures are built by operations which make no reference to meaning, discourse, or use. In one formulation, this notion is defined as syntax being arbitrary and self-contained with respect to meaning, semantics, pragmatics, and other factors external to language. Because of this, those approaches that adopt that assumption have also been called autonomist linguistics. The assumption of the autonomy of syntax is what most prominently distinguishes linguistic formalism from linguistic functionalism, and it is at the core of the debate between the two. Over the decades, multiple instances have been found of cases in which syntactic structures are actually determined or influenced by semantic traits, and some formalists and generativits have reacted to that by shrinking those parts of semantics that they consider autonomous. Over the decades, in the changes that Noam Chomsky has made to his generative formulation, there has been a shift from a claim of