Sebastian's soliloquy at the beginning of Act 4, Scene 3 is the first real insight that the audience gets into his character. The speech establishes Sebastian as a logical individual who is nonetheless choosing to enjoy the strange but wonderful aspects of his situation.
At the beginning of the soliloquy, Sebastian makes use of visual and tactile imagery:
Sebastian: This pearl she gave me, I do feel ’t and see ’t.
And though ’tis wonder that enwraps me thus,
Yet ’tis not madness.
Sebastian cannot be hallucinating, he reasons, because he can feel and see the pearl that Olivia gave him. Although he trusts his senses to a degree, he can't shake the feeling that something is wrong:
For though my soul disputes well with my sense
That this may be some error, but no madness,
Yet doth this accident and flood of fortune
So far exceed all instance, all discourse,
That I am ready to distrust mine eyes
And wrangle with my reason that persuades me
To any other trust but that I am mad—
Or else the lady’s mad.
He eventually convinces himself that, although his situation is quite unusual, neither he nor Olivia are insane:
Sebastian: Yet if ’twere so,
She could not sway her house, command her
followers,
Take and give back affairs and their dispatch
With such a smooth, discreet, and stable bearing
As I perceive she does.
Sebastian's conclusion comes about as a result of his use of logos: insane people cannot adequately run households; Olivia adequately runs hers; ergo, she cannot be mad.
This soliloquy serves as a sort of spiritual opposite to the "Is this a dagger which I see before me" soliloquy in Macbeth. In that play, Macbeth wonders whether the vision of the bloody dagger he sees is real or not, since he can see but not touch it, and ultimately decides that it is a hallucination brought on by his guilt. In Twelfth Night, by contrast, Sebastian concludes that he is not mad because he can both see and touch the pearl that Olivia has given him, but he remains suspicious that all is not as it seems.
