One can feel for a poetry reader in 1917 who, picking up a just-published collection by an evident newcomer, was floored by its opening lines:

Let us go then, you and I,
When the evening is spread out against the sky
Like a patient etherised upon a table...

This evokes an evening sky, which we naturally visualise, then tells us it is 'like' something which - as our reader, were it that time of day, might verify through a window - it does not visually resemble. It thwarts expectations of how similes work. So what is it doing? The sheer familiarity of the lines nowadays can blunt our alertness to the strangeness of Eliot's simile, which is not akin to that of Donne's similes, which is a matter of the unlikely provenance of what something is compared with, or where, in the case of his famous compasses, what is compared to their movement, parted lovers, is a concept and not something we have concretely visualised. If, instead of from habit gliding past it, we dwell upon what this evening sky is likened to, we may register the import of the simile as thematic, and precise: it signals the helplessly passive psyche of the speaker and, for such a patient is going under the knife, his fear of what he is in for. This is a stream-of-consciousness poem, in which every detail reveals something of the fictional mind apprehending it.

Persevering, our ...