Sonnet LIX (59)

If there be nothing new, but that which is
Hath been before, how are our brains beguiled,
Which, labouring for invention, bear amiss
The second burden of a former child.
O, that record could with a backward look,
Even of five hundred courses of the sun,
Show me your image in some antique book,
Since mind at first in character was done!
That I might see what the old world could say
To this composed wonder of your frame;
Whether we are mended, or whe'er better they,
Or whether revolution be the same.
O, sure I am, the wits of former days
To subjects worse have given admiring praise.

If there be nothing new (1): Compare Ecclesiastes 1.9: "The thing that hath been is that which shall be; and that which hath been done is that which shall be done; and there is no new thing under the sun."

beguiled (2): deceived.

labouring for invention (3): the birth pains of a new creation.

brains...former child (2-4): an extended metaphor describing the poet's brain as a mother's womb, labouring to bring forth original verse only to mistakenly write something ("second burden") that has already been written ("former child"), as if the mother had suffered two bouts of labour to give birth to the same child twice. 

record (5): memory.

five hundred courses of the sun (6): i.e., (by metonymy) five hundred years.

character (8): writing.

composed wonder of your frame (10): wonderful composition of your body (like the structure of a beautiful sonnet).

mended (11): improved.

whe'er (11): whether.

To subjects worse...praise (14): i.e., surely, no poet of old ever had a more worthy subject than the one I have.