"On His Blindness" is a Petrarchan sonnet, a lyric poem with fourteen lines. This type of sonnet, popularized by the Italian priest Petrarch (1304-1374), has a rhyme scheme of ABBA, ABBA, CDE, and CDE. John Milton wrote the poem in 1655. For more information about sonnets, see Origin of the Sonnet Form, below.

Theme

God judges humans on whether they labor for Him to the best of their ability. For example, if one carpenter can make only two chairs a day and another carpenter can make five, they both serve God equally well if the first carpenter makes his two chairs and the second makes his five. If one carpenter becomes severely disabled and cannot make even a single chair, he remains worthy in the sight of God. For, as Milton says in the last line of the poem, "they also serve who only stand and wait." 

Lines 3-6: Key to the Meaning

Lines 3 to 6 of the poem allude to the "Parable of the Talents" in Chapter 25 of the Gospel of Matthew, verses 14 to 30. In this famous parable, an employer who is going away for a time gives his three servants money in proportion to their ability to increase its value. He distributes the money in talents, a unit of weight used in ancient times to establish the value of gold, silver, or any other medium used as money. Thus, a Roman might pay ten talents of gold for military supplies or seven talents of silver for a quantity of food. In the "Parable of the Talents," the employer gives the first servant five talents of silver, the second servant two talents, and the third servant one talent. After the employer returns from the trip and asks for an accounting, the first servant reports that he doubled his talents to ten and the second that he doubled his to four. Both men receive promotions. The third servant then reports that he still has only one talent, for he did nothing to increase its value. Instead, he buried it. The employer denounces him for his laziness, gives his talent to the man with ten, and casts him outside into the darkness. 

Meter

All the lines in the poem are in iambic pentameter. In this metric pattern, a line has five pairs of unstressed and stressed syllables, for a total of ten syllables. The first two lines of the poem illustrate this pattern:

       1...........2........... 3............4............5
When I | con SID | er HOW.| my LIFE | is SPENT

       1................2............ 3...............4....................4
Ere HALF | my DAYS | in THIS | dark WORLD.| and WIDE

John Milton's eyesight began to fail in 1644. By 1652, he was totally blind. Oddly, he wrote his greatest works, Paradise Lost and Paradise Regained, after he became blind. Many scholars rank Milton as second only to Shakespeare in poetic ability. 

On His Blindness
By John Milton (1608-1674)

Text

When I consider how my light is spent¹
Ere half my days² in this dark world and wide
And that one talent³ which is death to hide
Lodged with me useless,⁴ though my soul more bent
To serve therewith⁵ my Maker, and present
My true account,⁶ lest he returning chide;
"Doth God exact⁷ day labor, light denied?"
I fondly⁸ ask. But Patience,⁹ to prevent
That murmur, soon replies,^. "God doth not need
Either man's work or his own gifts.¹⁰ Who best
Bear his mild yoke,¹¹ they serve him best. His state
Is kingly; thousands at his bidding speed,^.
And post¹² o'er land and ocean without rest;
They also serve who only stand and wait.¹³

Notes

1....light is spent: This clause presents a double meaning: (a) how I spend my days, (b) how it is that my sight is used up.
2....Ere half my days: Before half my life is over. Milton was completely blind by 1652, the year he turned 44.
3....talent: See Line 3: Key to the Meaning.
4....useless: Unused.
5....therewith: By that means, by that talent; with it
6....account: Record of accomplishment; worth
7....exact: Demand, require
8....fondly: Foolishly, unwisely
9....Patience: Milton personifies patience, capitalizing it and having it speak. 
10..God . . . gifts: God is sufficient unto Himself. He requires nothing outside of Himself to exist and be happy.
11. yoke: Burden, workload.
12. post: Travel.
. .
Examples of Figures of Speech

Alliteration: my days in this dark world and wide (line 2)
Metaphor: though my soul more bent / To serve therewith my Maker (lines 3-4). The author compares his soul to his mind.
Personification/Metaphor: But Patience, to prevent / That murmur, soon replies . . .  (lines 8-9).
Paradox: They also serve who only stand and wait.

Origin of the Sonnet Form

.......The sonnet form originated in Sicily in the thirteenth Century with Giacomo da Lentino (1188-1240), a lawyer. The poetic traditions of the Provençal region of France apparently influenced him, but he wrote his poems in the Sicilian dialect of Italian. Some authorities credit another Italian, Guittone d'Arezzo (1230-1294), with originating the sonnet. The English word "sonnet" comes from the Italian word "sonetto," meaning "little song." Some early sonnets were set to music, with accompaniment provided by a lute.
.......The Italian poet Petrarch (1304-1374), a Roman Catholic priest, popularized the sonnet form. Other popular Italian sonneteers were Dante Alighieri (1265-1321), Italy's most famous and most accomplished writer, and Guido Cavalcante (1255-1300). 
.......Petrarch's sonnets each consist of an eight-line stanza (octave) and a six-line stanza (sestet). The first stanza presents a theme, and the second stanza develops it. The rhyme scheme is as follows: (1) first stanza (octave): ABBA, ABBA; (2) second stanza (sestet): CDE, CDE (or CDC, CDC; or CDE, DCE). Sonnets written in this format by later poets came to be known as Petrarchan sonnets.
.......The sonnet form was introduced in England by Sir Thomas Wyatt (1503-1542) and Henry Howard, Earl of Surrey (1517-1547). They translated Italian sonnets into English and wrote sonnets of their own. Surrey introduced blank verse into the English language in his translation of the Aeneid of Vergil. Wyatt and Surrey sometimes replaced Petrarch's scheme of an eight-line stanza and a six-line stanza with three four-line stanzas and a two-line conclusion known as a couplet. Shakespeare adopted the latter scheme in his sonnets, and this form came to be known as the Shakespearean sonnet. 
.......Besides Shakespeare, well known English sonneteers in the late 1500's included Sir Philip Sydney, Samuel Daniel, and Michael Drayton.
.......In Italy, England, and elsewhere between the thirteenth and early sixteenth Centuries, the most common theme of sonnets was love. Sonnets in later times also focused on religion, politics, and other concerns of the reading public.

Study Questions and Essay Topics

1. Write an essay that provides examples of people who exemplify the last line of the poem. 
2. Research the life of John Milton. Then write an essay describing the methods he used to compose his poetry when he was blind.
3. Another famous man, Ludwig van Beethoven, composed great symphonies after he became deaf. Pete Gray, a baseball player, earned the right to play in the major leagues even though he had only one arm. Julius Caesar, an epileptic, became ruler of Rome. Do you believe that the human psyche has a way of compensating for a physical disability?