Macbeth's Dagger and Other Illusions

Hylas objects to Philonous' idealism by claiming that on his view there is no way to distinguish between veridical appearances and illusions. In other words, idealism implies that the danger than Macbeth sees before his eyes but cannot clutch is just as real as the dagger he uses to kill Duncan. Is this a valid objection? How successful is Philonous' response?