Polonius is saying that if you have to lie to find out some truth, then it is fine to lie. Polonius is asking Reynaldo to spy on Laertes and fish around for some dirt on Laertes. If Reynaldo has to act like a fake and be deceitful to find out some truth about Laertes that makes Laertes look bad, then that is fine with Polonius. This scene makes Polonius look like a fake and a fraud. For Polonius, it is acceptable to present a false front in public to hide the real ulterior and deceitful motives.
Polonius is saying that if you have to lie to find out some truth, then it is fine to lie. Polonius is asking Reynaldo to spy on Laertes and fish around for some dirt on Laertes. If Reynaldo has to act like a fake and be deceitful to find out some truth about Laertes that makes Laertes look bad, then that is fine with Polonius. This scene makes Polonius look like a fake and a fraud. For Polonius, it is acceptable to present a false front in public to hide the real ulterior and deceitful motives.