When comparing two languages, we begin by comparing lexical items of similar meaning or function in an attempt to derive a set of regular sound correspondences. Provided these allow us to predict additional etymologies without greatly expanding the set of correspondences, the odds that the two languages diverged from a common ancestor increase as the number of unmatched lexical items in each language decreases. Though the procedure is essentially inductive, there are only three possible reasons that the morphemes of similar function in two languages can regularly resemble one another in form: pure chance; borrowing; or mutual inheritance from a common source. Therefore, provided the number of etymologies is sufficiently large and the set of sound correspondences plausibly natural, we can reasonably conclude that a common origin existed even when it can no longer be observed directly.
There is still scholarly disagreement as to whether the proto-Korean-Japanese reconstruction initiated in 1966 by Samuel E. Martin and extended by John B. Whitman in 1985 has reached this level of inductive certainty. One of the reasons for this is that Whitman included several etymologies that are probably better regarded as products of borrowing. I summarize criteria that enable us to identify borrowings from Old Korean into Old Japanese in the interval between the Yayoi and Nara periods and discuss some examples. (I also call attention to some doubtful claims of borrowed Korean words in the literature.) Not only are these early borrowings interesting in their own right, but the fact they do not significantly reduce the amount of other closely matching lexical material in Korean and Japanese casts doubt on theories of massive borrowing during the critical period.
J. Marshall Unger is a Professor of Japanese at the Ohio State University, where he chaired the Department of East Asian Languages and Literatures from 1996 to 2004 having previously chaired similar departments at the University of Hawai’i and University of Maryland (1988–96). He has published on the history of Japanese, the teaching of Japanese as a second language, and the writing systems, script reforms, and impacts of computerization in Japan, China, and Korea. Though he continues to work in these areas, his latest project is a translation and commentary on twenty-six selected problems from Saijō-ryū sanpō kantsū jutsu (Inductive Calculation Techniques of the Saijō School) by the famous mathematician Aida Yasuaki (1747–1817).