In terms of its theme and emotional dynamics 15 is one of Shakespeare’s simpler sonnets, but it has enormous power. We feel this from the very start:

When I consider everything that grows
Holds in perfection but a little moment,

The power is partly in the sheer vastness of that initial assertion and partly in the explosive way vastness is contrasted with littleness in the second line.  This creates an extreme sense of suspense – where can he be going with this? – which is in itself intensely involving. The third line makes a similar contrast but far from tamely repeating the idea Shakespeare speeds up its development (line 3 covers the ground of both the first two lines), makes it more extreme (not contrasting something vast with something little but hugeness with emptiness) and makes it not a process of becoming but an instantaneous state of being. Developing line 3, line 4 shoots off in a new direction as it wonderfully combines evocation of the stellar space embracing Earth – projecting our imaginations as far outwards as they can go – with an intimate impression of people whispering to each other in a theatre audience:

That this huge stage presenteth naught but shows
Whereon the stars in secret influence comment

His mind moves among ideas at dazzling speed. What’s strikingly at variance with the shibboleths of Ezra Pound and the imagist movement, and I believe much creative writing teaching even now, is how abstract these ideas are and how abstractly they’re presented – at least in a certain sense.  I think line four is the first in which we have anything that could really be called an image. At a stretch you might say ‘stage’ and ‘shows’ imply images of performance, but neither word is given any kind of concrete content until line 4, and the comparison of life to a play is in itself fairly common in the writing of the time. ‘Huge’ I suppose gives a feeling of concreteness partly because of the heavy emphasis that falls on it as a strongly stressed syllable falling where we’d normally have an unstressed one.

Concreteness in this poem comes less from images – even the brilliant one in line 4 – than from the drama of unfolding thought, the sense of being in the immediate presence of a mind simultaneously having and sharing ideas, from the first three quiet words through the sudden expansion in the second half of line one. If free use of abstract ideas gives the poem its reach, the intimately personal, circumstantial nature of the opening address to his beloved – ‘When I consider’ – gives it a vital concrete grounding and an emotional impetus that’s sustained by the rapid unfolding of the rest of the poem.