The poem plays on the cliché "thinking outside the box" and takes it to an odd extreme: the speaker's mind actually leaves. What's left behind is a body, a presence, with "little to do" except simply exist. The line about schooling a school in being is a strange, slightly comic image. It suggests teaching something that already knows how to be, like fish taught how to swim. The man "buried in being" sounds both peaceful and a bit like a grave.
The questions that follow, "Brain or pot? Water or wine?", feel like a shrug. The speaker seems unsure what fills him now, whether it's thought, a high, or something plainer or more celebratory. Judging by the final line, the answer hardly matters. "Other things make a kind bed" suggests comfort can be found somewhere other than in thinking. The mood is dry, playful, and a little drowsy: a relief at being mindless, with a faint hint of loss underneath.