What are we trying to explain?
	l Almost all organisms communicate. l However, human language biologically unique. ¡ Learned through observation without negative evidence, or reliable feedback from limited experience. ¡ Infinitely expressive using finite means. ¡ Syntactic structure: autonomous system mediating signals and meanings. ¡ Dynamic: language changes on many timescales. ¡ Constrained cross-linguistic variation. e.g., word-order universals. ¡ Mixture of regular rule-based regularity and irregularity.