Milton's major preoccupation in 'On his blindness' is blindness itself which prevents him from exploiting to the full :he talent that God has endowed him with. That is his affliction. Starting on a note of deep reflection, he laments his state of blindness: "how my light is spent/ere half my days, in this dark world". He regrets his loss of sight, which has not allowed him to serve God and humanity as he would have wished. For him, this talent (his sight) "is death to hide/Lodeg with me useless".
Though my soul more bent/To serve therewith my maker." His fear is that he risks God's chastisement on his returning to Him without accomplishing much. Alluding to the biblical parable of the talent, the poet is worried that he may suffer the fate of the "unprofitable" servant who failed to utilize the talent given him by his master and was consequently condemned. However, he rationalizes that if it were God's wish that he achieved something in life, God would not have denied him :he necessary instrument with which to accomplish such a task. He asks rhetorically, "doth God exact day labour, light denied?" Upon further reflection, he quickly counsels patience "to prevent that murmur."
Again, this is an allusion to the same "unprofitable servant" in the Bible who grumbles about the talent given to him and finds excuses to explain away his failure to trade and profit by it. For Milton, it is neither man's work nor his services that God requires, but that of those who, through patience have learnt to bear their yoke without murmur. Such people, to him, serve God best because they have a lot to teach humanity. He asserts that, after all, God is sovereign and royal.
There are thousands of servants 'at his bidding speed' who run his errands, both over land and sea without rest. This recognition allows Milton to celebrate the spirit over the flesh and mind over matter, as well as man's immense capacity to come to terms with the problems that beset him. Milton reiterates the point that man's effort alone is not enough to please God, rather, it is his resolve to refuse to bow to adversity and 'bear his mild yoke' for "they also serve who only stand and wait".