Final answer:
Polonius attributes Hamlet's madness to his unrequited love for Ophelia, based on his observations and an intercepted love letter.
Step-by-step explanation:
In Hamlet by William Shakespeare, Polonius believes that the source of Hamlet's madness is his unrequited love for Polonius's daughter, Ophelia. Polonius asserts this theory to King Claudius, convinced that Hamlet's erratic behavior and apparent lunacy are the result of Ophelia rejecting his affections. This conclusion is drawn from observations and an intercepted love letter from Hamlet to Ophelia. Polonius's interpretation of Hamlet's state contributes to the complex unfolding of the play's themes of madness, love, and deceit.