In Alexander Pope's An Essay on Criticism, he is speaking about the "art" of being a good critic. This piece is "one of the best known discussions of literary criticism" in English. Pope was only twenty-two years old when he wrote it.
The first comment Pope makes about learning is that only those who have excelled in writing should teach others. If one has not mastered the skill, he (or she) should not try to instruct or criticize others.
Let such teach others who themselves excell,
And censure freely who have written well.
Pope also warns of "false Learning," saying that it destroys "good Sense," but that some people are confused by the many schools of thought that exist.
So by false Learning is good Sense defac'd.
Some are bewilder'd in the Maze of Schools...
Pope states that a little learning is a dangerous thing. One must be well-versed in that which he hopes to stand as an expert. To depend only on a little learning (taking "shallow" sips) will not yield the positive result that comes of "drinking deeply." One should drink at the Pierian Spring, the spring of knowledge in Greek mythology that "fed" the Muses. One must learn as much as possible.
A little Learning is a dang'rous Thing;
Drink deep, or taste not the Pierian Spring:
There shallow Draughts intoxicate the Brain,
And drinking largely sobers us again.