Grammatical correspondences provide proof, and they alone prove rigorously, but only if one makes use of the details of the forms and if one establishes that certain particular grammatical forms used in the languages considered go back to a common origin. Correspondences in vocabulary never provide absolute proof, because one can never be sure that they are not due to loans (Meillet 1954: 27).
Correspondence in vocabulary and regular phonetic correspondence can be between any randomly chosen languages. For instance it is possible to find some regular correspondence between Japanese and Cantonese and even “prove” their relationship: boku Japanese personal pronoun “I” used by males – Cantonese buk “servant”, “I”; Japanese bō “stick” – Cantonese baang “stick”; Japanese o-taku “your family”, “your house” or “your husband” – Cantonese zaak “house”; Japanese taku “swamp” in compounds – Cantonese zaak “swamp”; Japanese san “three” – Cantonese sam; Japanese shin “forest” used in compounds – Cantonese sam “forest”; Japanese roku “six” – Cantonese lük; Japanese ran “orchid” – Cantonese laan “orchid”. If there would be no other languages of so called Buyeo3 stock4 and no languages of Chinese stock we would have no ability to single those words as items borrowed from Southern Chinese dialects since they have same regular and wide use as well as words of Japanese origin. In the case of Japanese and Cantonese we know history of correspondent stocks rather well and have many firm evidences that Japanese isn’t a relative of Chinese stock.
If someone thinks that the example of Japanese and Cantonese is just a weird joke, then everyone can take a look at the procedure that was used by Greenberg in order to prove that Waikuri language belonged to Hokan stock5: the conclusion was based on comparison of FOUR (!) words only (Poser, Campbell 1992: 217 – 218). Also we should keep in mind that Greenberg actually didn’t care much about precise phonetic correspondence and superficial likeness was rather sufficient for him.
Pic.1. Map representing location of hypothetical Hokan stock (blue) and Waicuri language (red)
Phonetic correspondences themselves can be even between completely unrelated languages and so a stock can’t be proved by regular correspondences, but regular correspondences should be proven by existence of a stock since true regular phonetic correspondences exist only inside stocks.