  Suppose that the primary linguistic data were impoverished in all the ways that nativists claim and suppose, too, that children know a  bunch of things for which there is no evidence available —  suppose, as Hornstein and Lightfoot (1981:9) put it, that  “[p]eople attain knowledge of the structure of their language  for which no evidence is available in the data to which they are exposed as children.” What follows from this is that there  must be constraints on the learning mechanism: children do not  enumerate all possible grammatical hypotheses and test them against  the data. Some possible hypotheses must be ruled out a  priori. But, critics allege, what does not follow from  this is any particular view about the nature of the requisite  constraints. (Cowie 1999: ch.8.) A fortiori, what does not  follow from this is the view that Universal Grammar (construed as a  theory about the structural properties common to all natural  languages, per Terminological Note 2 above) is inborn.