In Hamlet, what does Lord Polonius' statement, "Your bait of falsehood takes this carp of truth," mean?
Lord Polonius says this to Reynaldo, his servant, in Act 2 scene 1. He is sending Rynaldo to Paris to give him some notes and money, but more importantly, to spy on his son. Laertes has been given permission by his father and king Claudius to leave the country and travel to France. The purpose of Lord Polonius' instruction is to teach Reynaldo how to be deceptive so that he may obtain information about Laertes' actions and his whereabouts. It is clear that he wants Reynaldo to learn as much as possible about his son. It is clear that Polonius is not only devious but also quite meddlesome in this regard.
He, for example tells Reynaldo that he should not enquire about Laertes directly but should act as if he was only a passing acquiantance and that he would like to know more about him since he is looking for him. He must ask whether Laertes was seen in the company of other Danes and what kind of company they were.
He encourages Reynaldo to mention a few negative things which he knows about Laertes, but that he should not slander his good name. He must speak as if what he mentions is part of the nature of unbridled youth and since Laertes is young, it would be normal for him to behave as he does. Polonius believes that in this way Reynaldo would be able to draw further information from the one/s he speaks to.
It is essential however, that Reynaldo never makes it obvious that he is close to Laertes or that he knows him intimately. He must seek information in a roundabout way, by even lying about things, which would encourage his listener to add to what he has said about Laertes.
In this sense then, Lord Polonius uses the following metaphor:
Your bait of falsehood takes this carp of truth:
And thus do we of wisdom and of reach,
With windlasses and with assays of bias,
By indirections find directions out:
So by my former lecture and advice,
Shall you my son. You have me, have you not?
What he means is that whatever made-up stories Reynaldo has created about Laertes, would act as bait to encourage others to provide further information, i.e. the truth, and so will he be able to establish exactly what Laertes is up to. By using deception, Reynaldo would be able to learn much more about Laertes' comings and goings and general conduct than he would otherwise.
It is apparent that Polonius does not entirely trust his son, even though he had advised him about proper and honourable behaviour when he is in France. he made it a point that Laertes understood what he, as father, expected of his son. We also learn here that Polonius is somewhat of a hypocrite. He expects Laertes to display moral excellence and be at his best behaviour, whilst he encourages Reynaldo to tell lies and be misleading, supposedly to get to the truth.