I discuss this very topic in our free "Lit 101" classes on Hamlet.
Here is Lesson 9: "Polonious in Spin Control"
The curtain rises on Polonius slipping money and a note to Reynaldo, his personal servant. Soon it becomes clear that this Polonius differs substantially from the purveyor of parental wisdom of Act 1. Reynaldo is instructed to do two things: 1) try to observe Laertes engaged in unseemly behavior, and more importantly 2) to discover his son’s reputation around Paris. But, Polonious warns, he mustn’t be too obvious about it and he painstakingly instructs Reynaldo in the finer points of subterfuge. Should anyone ask why Reynaldo is interested in Laertes’ activities, Polonius advises Reynaldo to act distant and claim only that he knows “his father and his friends” (2.1.16).
It only gets worse. Polonius emphatically tells Reynaldo that he wants real dirt, not just youthful indiscretions like drinking and gambling or even sexual liaisons. No, he wants more. In his pressing for “facts,” Polonius reveals much more of his own character than he could ever have intended: “See you now,” he says, “Your bait of falsehood take this carp of truth; And thus do we of wisdom and of reach , / … / By indirections find directions out. / So by my former lecture and advice / Shall you my son” (2.1.69-75). An accomplished liar knows that the believability of a falsehood is greatly enhanced by a bit of reality at its core; a “carp of truth” baiting the fishook that will reel in the prey.
What we don’t know is exactly why Polonius is so suspicious of his son. Perhaps he is jealous of his son’s freedom or maybe he is worried that Laertes’ behavior away from home will affect him personally in the eyes of King Claudius. Or, more likely, he fears that Laeretes will put two and two together and figure out the nefarious plot that resulted in the death of the King, the installation of Claudius, and subsequently, Polonius’ new position as Claudius’ right-hand man. Just as Claudius wanted Hamlet to close by to keep his nephew in check, Polonius too is uncomfortable with this son so far away, safe from the vortex of his spin.
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This is a classic! Of course, parents want to know what their children are up to when away from home at school or other events. However, I am not sure that sending a spy is the best way to do it. In addition, he's asking the spy to probe Laertes' friends with questions about his drinking habits, relations with women, spending habits, and other social aspects which may effect his studying. In essence, the spy is to lie about Laertes to find out the truth through the objections of Laertes' friends, but not to malign Laertes' character so much that he will no longer have any friends. A few questions that come to mind are: What if no one corrects the spy? Will others who do not know Laertes and are privy to the "act" believe that Laertes is such a horrible person? Why is Polonius willing to put his son through this instead of just asking Laertes himself?