Introduction. [The introduction has yet to be written.]

Sonnets 18, 19, 20, 21, and 23 were numbered XV., XVI., XVII., XVIII., and XIX., respectively in Poems (1673).

Sonnet 21. All but the first four lines of this sonnet appear on a sheet of the Trinity MS along with Sonnet 22.

Cyriack. Cyriack Skinner was the grandson of Sir Edward Coke, who served as Chief Justice of the King's Bench from 1613-16 and authored the famous The Institutes of the Law of England. He served Milton as a reader and amanuensis.

Themis. Goddess of justice.

Let Euclid rest. Cyriack Skinner may have learned about ancient mathematicians while a pupil at Milton's school on Aldersgate Street.

what the Swede intend. Cyriack Skinner's study of international politics may have lead to an interest in the campaign against the Poles conducted by Charles X of Sweden in 1655. The poem echoes Horace's Odes 2.11.1-6 and 3.8.17, in which the speaker tells his friend to choose recreation over study, because youth is too fleeting not to have fun.

Heav'n a time ordains. In Ecclesiastes 3:1 states "To every thing there is a season."