The heaven of Mercury may be compared to Dialectics because of two properties: for Mercury is the smallest star of heaven...; the other property is that in its passage it is veiled by the rays of the sun more than any other star.

These two properties are found in Dialectics, for Dialectics is less in substance than any other science [...]; and its passage is veiled more than that of any science, in that it proceeds by a more sophistical and polemical mode of argument than any other.

Convivio II, XIII, 11-12