The poem begins with a playful idea: the best pocket you could carry is a simple head. A head holds things, ideas and memories, without bulk or fuss. But the speaker quickly turns inward. Instead of just using the mind, he starts "measuring the measurer," examining the very thing that does the judging.
That's where the title, "Interrogation," starts to make sense. The speaker questions his own mind, talking with it "about discourse," so thought ends up studying thought. The answer is a little comic and a little frustrating. The mind, which he calls "him" as if it were a separate person, turns out to be "not so thorough a philosopher." It won't accept the speaker's "rule," his way of measuring.
The mood is dry, wry, and thoughtful. There's no anger, just mild amusement at a mind that can't be pinned down by its own standards. The speaker seems to be arguing with himself and not quite winning, which makes the poem feel intimate and gently humorous.