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[The] comparative method is only one of the procedures of historical linguistics. With the method of internal reconstruction, one deals only with material in one language; that means one can push back beyond a proto-language even if there are no related languages. And through examining everyday language one often finds historical relics, such as umlaut in man:men, [and] ablaut in sing:sang:sung:song, because children learn such parts of language before they master the rules, such as making plurals by adding -s. And then there is the current typology, as in the article [W.P. Lehmann and Cristina Rinaldi, "Human Language and the Functioning of the Brain," Logos and Language 4 (1), 2003, pp. 17-33]. So our "limits" would be much earlier than [those applying to the comparative method].
W. P. L.