And every fair from fair sometime declines, explain!

These two lines have been taken from “Sonnet-XVIII”, composed by William Shakespeare, an Elizabethan poet and dramatist. Here is a reference to the law of mutability. In other words, here the speaker of the poem reminds a universal truth that all beautiful things of this world are subject to destruction, either by accident or by the law of nature. 

These lines occur at the end of the second quatrain of the poem. The speaker, in a move to prove that his friend’s beauty is more temperate than summer’s day, makes this general statement about the mutability of this world. Summer has lots of negative qualities. The temperature changes frequently. Its duration is short. Its winds destroy blooming roses. The speaker is of the view that not only summer but also all beautiful things in nature are subject to change. 

It is because the law of nature causes change in all things, But the youth is not subject to any law of nature, His beauty will not fade. He will remain the same because he has been immortalised in these verse lines. These lines add universality fo the poem by referring to the law of nature. Here the word “fair” is a synecdoche, By using an adjective the poet has meant the thing. This figure of speech has added charm and felicity to the poem.