Act 1, Sc. 5, lines 174-185: What does the audience know that Hamlet's friends do not about his motives?

Hamlet: And therefore as a stranger give it welcome.

There are more things in heaven and earth, Horatio,

Than are dreamt of in your philosophy. But come.

Here, as before, never, so help you mercy,

How strange or odd some'er I bear myself

(As I perchance hereafter shall think meet

To put an antic disposition on)

That you, at such times seeing me, never shall,

Or by pronouncing of some doubtful phrase,

As "Well, well, we know," or "We could an if we would."

Or "If we list to speak," or "There be an if they might,"

Or such ambiguous giving-out, to note

That you know aught of me---this do swear,

So grace and mercy at your most need help you.