Although the earliest surviving Greek inscriptions written in an alphabet date from the eighth century B.C., the fact that the text of these Greek inscriptions sometimes runs from right to left and sometimes from left to right indicates that the Greeks adopted alphabetic writing at least two centuries before these inscriptions were produced. After all, the Greeks learned alphabetic writing from the Phoenicians, and presumably, along with the alphabet, they also adopted the then-current Phoenician practice with respect to the direction of text. And although Phoenician writing was originally inconsistent in direction, by the eighth century B.C. Phoenician was consistently written from right to left and had been for about two centuries.

In the argument given, the two portions in boldface play which of the following roles?