Why does Pope state that the proper study of mankind is man?
As well as being an important poem expressing Pope's own moral beliefs, Pope's "Essay on Man" is in many ways a reaction to Milton's "Paradise Lost." Pope begins Epistle II with the couplet:
Know then thyself, presume not God to scan;
The proper study of mankind is man.
This couplet asserts several anti-Miltonic ideas. First, it is a direct response to Milton's claim that "Paradise Lost" serves to "justify the ways of God to man." Pope is arguing that God is perfectly capable of doing his own job and that it is arrogant of a human poet to presume to fully comprehend God and to, in essence, usurp God's position by rewriting the Bible in verse. Instead, Pope suggests that the business of the human writer to to deal with human matters. The phrase "Know then thyself" suggests Socrates as a model of the scope of human inquiry.
Next, Pope is opposing his own Enlightenment rationality and toleration to what he sees as Milton's narrow, dogmatic Puritanism. In the wake of a period of great religious wars and upheavals that had caused untold suffering in Britain and Europe, Pope's "Essay on Man" is a model of an Augustan tolerance which tries to find universal human moral values rather than narrow points of theological difference.