Cognitive grammar deals mainly with the semantic content of constructions, and its central argument is that conceptual semantics is primary to the degree that form mirrors, or is motivated by, content. Langacker argues that even abstract grammatical units like part-of-speech classes are semantically motivated and involve certain conceptualizations. Radical construction grammar William A. Croft's radical construction grammar is designed for typological purposes and takes into account cross-linguistic factors. It deals mainly with the internal structure of constructions. Radical construction grammar is totally non-reductionist, and Croft argues that constructions are not derived from their parts, but that the parts are derived from the constructions they appear in. Thus, in radical construction grammar, constructions are likened to Gestalts. Radical construction grammar rejects the idea that syntactic categories, roles, and relations are universal and argues that they are not only language-specific, but also construction specific. Thus, there are no universals that make reference to formal categories, since formal categories are language- and construction-specific. The only universals are to be found in the patterns concerning the mapping of meaning onto form. Radical construction grammar rejects the notion of syntactic relations altogether and replaces them with semantic relations. Like Goldbergian/Lakovian construction grammar and cognitive grammar, radical construction grammar is closely related to cognitive linguistics, and like cognitive grammar, radical construction grammar appears to be based on the idea that form is semantically motivated. Embodied construction grammar Embodied construction grammar (ECG), which is being developed by the Neural Theory of Language (NTL) group at ICSI, UC Berkeley, and the University of Hawaii, particularly including Benjamin Bergen and Nancy Chang, adopts the basic constructionist definition of a grammatical construction, but emphasizes the relation of constructional semantic content to embodiment and sensorimotor experiences. A central claim is that the content of all linguistic signs involves mental simulations and is ultimately dependent on basic image schemas of the kind advocated by Mark Johnson and George Lakoff, and so ECG aligns itself with cognitive linguistics. Like construction grammar, embodied construction grammar makes use of a unification-based model of representation. A non-technical introduction to the NTL theory behind embodied construction grammar as well as the theory itself and a variety of applications can be found in Jerome Feldman's From Molecule to Metaphor: A Neural Theory of Language (MIT Press, 2006). Fluid construction grammar Fluid construction grammar (FCG) was designed by Luc Steels and his collaborators for doing experiments on the origins