Which. — Rather, Inasmuch as they.
The work of the law. — The practical effect or realisation of the law — written in their hearts as the original Law was written upon the tables of stone, (Comp. Jeremiah 31:33; 2 Corinthians 3:3.)
Also bearing witness. — Or, witnessing with them, as margin. There is a double witness; their actions speak for them externally, and conscience speaks for them internally.
The mean while. — Rather, literally, as margin, between themselves — i.e., with mutual interchange, the thoughts of the heart or different motions of conscience sometimes taking the part of advocate, sometimes of accuser.
This seems, on the whole, the best way of taking these two words, though some commentators (among them Meyer) regard this quasi personification of “the thoughts” as too strong a figure of speech, and take “between themselves” as referring to the mutual intercourse of man with man. But in that mutual intercourse it is not the thoughts that accuse or defend, but the tongue. The Apostle is speaking strictly of the private tribunal of conscience.