MIRANDA 'Tis far off And rather like a dream than an assurance That my remembrance warrants. (1.2.56-58)
What is the relationship between dreams and memory? Prospero has a handy habit of putting folks to sleep when he's up to some other sorcery—memories don't seem trustworthy in this environment.
Act 2, Scene 2
STEFANO What's the matter? Have we devils here? Do you put tricks upon's with savages and men of Ind? Ha? I have not scaped drowning to be afeard now of your four legs, for it hath been said "As proper a man as ever went on four legs cannot make him give ground," and it shall be said so again while Stefano breathes at' nostrils. (2.2.58-64)
To the Shakespearean audience, the wild tales of the Bermudas and other newly found and colonized places was as good as fantasy, though the places were real. Kind of like us talking about Mars.
Act 3, Scene 2
CALIBAN As I told thee before, I am subject to a tyrant, a sorcerer, that by his cunning hath cheated me of the island. (3.2.46-48)
Is Caliban wrong? He says what he sees to be reality, and his reality is no less credible than Prospero's, a man who is also a victim of usurpation (each had his land and title taken away).
Act 3, Scene 3
Sebastian and Antonio
SEBASTIAN A living drollery! Now I will believe That there are unicorns, that in Arabia There is one tree, the phoenix' throne, one phoenix At this hour reigning there. ANTONIO I'll believe both; And what does else want credit, come to me And I'll be sworn 'tis true. Travellers ne'er did lie, Though fools at home condemn 'em. GONZALO If in Naples I should report this now, would they believe me? If I should say, I saw such islanders— For, certes, these are people of the island— Who, though they are of monstrous shape, yet note Their manners are more gentle, kind, than of Our human generation you shall find Many, nay, almost any. (3.3.26-41)
Once one fantasy is proven true, the seer can no longer trust what he believes to be reality.
ARIEL You are three men of sin, whom Destiny, That hath to instrument this lower world And what is in 't, the never-surfeited sea Hath caused to belch up you, and on this island, Where man doth not inhabit, you 'mongst men Being most unfit to live. I have made you mad; And even with such-like valor men hang and drown Their proper selves. (3.3.70-71)
Anyone whose conscience yelled at him for being a traitor would see this reality as a punishment for his wrongdoing. Sebastian and Antonio have such warped views of reality that only Alonso actually benefits from the reality check of the harpy. The other men have no consciences worth noting, and feel their reality is beyond moral consequence.
Act 5, Scene 1
PROSPERO You do yet taste Some subtleties o' th' isle, that will not let you Believe things certain. (5.1.139-141)
Prospero is playing false here—he knows it is his magic, and not the island, that plays with others' views of reality. Or does Prospero really believe that all his magic is helped by the magic inherent in the island?
King Alonso of Naples
ALONSO If this prove A vision of the Island, one dear son Shall I twice lose. SEBASTIAN A most high miracle! (5.1.205-208)
Sebastian, seeing that the Prince lives, realizes that his view of reality was swayed by Antonio's evil persuasion; he lets go of the idea of usurpation as easily as he came by it.
ALONSO Whe'er thou be'st he or no, Or some enchanted trifle to abuse me (As late I have been) I not know. Thy pulse Beats as of flesh and blood; and since I saw thee, Th' affliction of my mind amends, with which I fear a madness held me. This must crave, An if this be at all, a most strange story. Thy dukedom I resign, and do entreat Thou pardon me my wrongs. But how should Prospero Be living and be here? (5.1.123-133)
Alonso didn't have a strong sense of reality to begin with—he knows he is easily swayed by flattering words, and he now realizes he can no more trust his eyes than his easily influenced intuitions.
Sebastian and Antonio
ADRIAN The air breathes upon us here most sweetly. SEBASTIAN As if it had lungs and rotten ones. ANTONIO Or as 'twere perfumed by a fen. GONZALO Here is everything advantageous to life. ANTONIO True, save means to live. SEBASTIAN Of that there's none, or little. GONZALO How lush and lusty the grass looks! How green! ANTONIO The ground indeed is tawny. SEBASTIAN With an eye of green in 't. ANTONIO He misses not much. SEBASTIAN No, he doth but mistake the truth totally. (2.1.49-60)
Reality reflects the nature of the speaker. This is not because reality is false, but because perspective is paramount.