Abstract: (from the book) Begins by distinguishing among 3 metaphors of acquisition: the copy, hypothesis-testing, and trigger metaphors. After outlining these 3 positions, the distinction among direct and indirect positive evidence and direct and indirect negative evidence and their roles in languages acquisition under the hypothesis-testing and trigger theories are reviewed. Then, experimental work on the role of input in acquisition is reviewed. The author distinguishes among 3 types of studies that have been performed to determine the role of the environment in acquisition. It is concluded on the basis of this work that neither input nor reply studies show any correlations between linguistic activity in the child’s environment and progress in acquisition.