[…] the comparer must be thoroughly and equally versed in the materials of both sides of the comparison.
- Whitney (1875), a pag.314

These two difficulties [borrowed words, accidental correspondences between words] impose upon the comparer of languages the necessity of increased caution in his work, and warn him against over-hasty conclusions. An instance or two, or a few instances, of verbal correspondences are not sufficient to prove anything. But accidental resemblances have their limit; and it is in general possible to distinguish borrowed material, so as not to be misled by it into false inferences. The linguist looks to see both how many and how close the asserted correspondences are, and in what part of the vocabulary they are found.
- Whitney (1875), a pag.170