Why do we say "A little knowledge is a dangerous thing"?

Meaning:

Incomplete knowledge of a subject can be more dangerous than no knowledge.

Background:

Way back in the first century B.C. Publilius Syrus wrote, "Better be ignorant of a matter than half know it." Publilius, a Syrian (Aramean), was brought to Italy as a slave, but won the favor of his master who both freed and educated him.

It took a further eighteen centuries before Alexander Pope (1688-1744) coined the proverb that is close to what we use today: "a little learning is a dang'rous thing" in his 1711 poem, An Essay on Criticism. This same work is also the earliest known source of other popular expressions including: “To err is human; to forgive, divine” and “For fools rush in where angels fear to tread.”)

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