A pocket is a humble thing, and the poem begins by making the head one. "The best and most / commodious pocket / is a simple head" reads like a proverb: carry your knowledge lightly, in a plain, roomy space, not in a crammed library. The rest of the poem then sets about testing that advice. Its subject is what happens when the mind turns to examine itself, and its answer is gently comic. The instrument doing the examining is the thing being examined, so the examination never quite works.
The title, "Interrogation", prepares us for something harsher than what follows. The questioning is a polite conversation, in which the speaker is "discoursing with the mind / about discourse." The mind is treated as a separate person, someone to sit across from and cross-examine. Yet the subject of their talk is talk itself, so the exchange folds back on itself. The same loop appears in "measuring the measurer," where the repeated word turns the sentence into a small hall of mirrors. The speaker uses a measure on the very thing that makes measurements.
The poem's sounds support this. "Discoursing" and "discourse" echo each other across a line break, as though the thought is chasing its own tail. The first stanza leans on soft, rounded consonants, the *m* of "most" and "commodious" and the *p* of "pocket" and "simple," which gives the opening a cozy, padded feel. "Commodious" is also the longest, most ornate word in the poem, an old-fashioned term meaning roomy or convenient. It sits oddly beside the plainness it praises. The vocabulary of "commodious" and "discoursing" has the flavor of an earlier, essayistic style of writing, and the poem seems amused by its own formality.
Formally there is little to hold on to, which suits the subject. The poem is three tercets of free verse, with no rhyme, no punctuation and no capital letters. Nothing marks where a thought ends, so the lines read like thinking out loud. The first break after "most" is a small trick: the line hangs on a superlative that is not yet attached to anything, and "commodious" arrives a beat later. The second stanza moves into the past tense ("I took to measuring"), and the poem shifts from a general maxim to a personal experiment. The last stanza delivers the result.
That result is the poem's best joke. The mind, now called "him," turns out to be "not so thorough a philosopher / not being satisfied with my rule." The logic is delightfully crooked. A real philosopher would be dissatisfied by definition, since dissatisfaction is the trade. The speaker, though, counts the mind's refusal to accept his "rule" as a failing. "Rule" works on two levels. It means a principle or method, and it also means a ruler, the measuring stick left over from "measuring the measurer." The mind will not be held up against it. Whether the fault lies with the mind or with the stick is left open, and the speaker's smugness is part of what the poem gently mocks.
The ending also quietly overturns the opening. The "simple head" is the ideal, but the head that turns up in the interrogation is anything but simple. It keeps asking questions and rejecting answers. A pocket can hold things, but it cannot turn itself inside out to inspect its own lining. The poem seems to suggest that self-knowledge by argument is a game the mind is too restless to lose or win.
What makes the poem worth reading is how much it fits into nine short lines without becoming heavy. It takes a very old puzzle, how the mind can study itself with its own tools, and treats it with dry wit. It trusts an everyday image, the pocket, and a pun on "rule" to do work that a longer philosophical argument would struggle with. The last line stops without a full stop, which fits: the speaker has no verdict, only a mind that will not sit still to be measured.