Alexander Pope's An Essay on Man concludes with these lines:
That, urg'd by thee, I turn'd the tuneful art
From sound to things, from fancy to the heart;
For wit's false mirror held up nature's light;
Shew'd erring pride, whatever is, is right;
That reason, passion, answer one great aim;
That true self-love and social are the same;
That virtue only makes our bliss below;
And all our knowledge is, ourselves to know.
It seems that in the first three lines of this stanza, Pope is speaking of his poem itself. Presumably "the tuneful art" is the art of poetry, and he's saying that his goal in this poem went beyond "sounding nice" towards more substantial things (i.e. his philosophy of how to live). The last five lines of the stanza seem to list the five key tenets of his philosophy, which are presumably informed by "nature's light" (the Enlightenment-era idea of "light of nature").
But then what does he mean in the third line by "wit's false mirror"? Presumably this is a reference to his own famous wittiness. Why does he think his witty poetry a false mirror rather than a true mirror to the "light of nature"?