Spying is an interesting theme in William Shakespeare's Hamlet because so many of the characters are engaged in it.
By far the worst offender is Polonius. He is the king's right-hand man, so we assume he has had some practice at it and perhaps some need for it. However, he spies even on his own children. Polonius sends Reynaldo, his servant, to spy on his son, Laertes, while Laertes is away at college. Even worse, he tells Reynaldo that he is free to spread some lies about Laertes as long as they help him discover the truth.
Polonius and Claudius both spy on Hamlet and Ophelia:
At such a time I'll loose my daughter to him:
Be you and I behind an arras then;
Mark the encounter: if he love her not
And be not from his reason fall'n thereon,
Let me be no assistant for a state,
But keep a farm and carters.
He rather stakes his life and livelihood...
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