What is Macbeth saying in this quote?

This supernatural soliciting Cannot be ill, cannot be good: if ill, Why hath it given me earnest of success, Commencing in a truth? I am thane of Cawdor: If good, why do I yield to that suggestion Whose horrid image doth unfix my hair And make my seated heart knock at my ribs, Against the use of nature? Present fears Are less than horrible imaginings: My thought, whose murder yet is but fantastical, Shakes so my single state of man that function Is smother'd in surmise, and nothing is But what is not.