Macbeth and Banquo come upon the weird sisters, the three witches, who offer predictions for both men. They greet Macbeth as thane of Glamis, thane of Cawdor, and "king hereafter." Macbeth is surprised. He knows that he is already thane of Glamis, but the thane of Cawdor and the king are both still alive. He cannot fathom how he, then, could attain either title:
By Sinel's death I know I am thane of Glamis;
But how of Cawdor? the thane of Cawdor lives,
A prosperous gentleman; and to be king
Stands not within the prospect of belief,
No more than to be Cawdor.
Shortly thereafter, Ross and Angus arrive and announce that the king is going to give Macbeth the title of thane of Cawdor. At this point, Macbeth begins speaking to the audience in a series of asides (although not, technically, a soliloquy, where the speaker would be alone on stage, the aside allows the speaker to state his thoughts out loud to the audience while other character onstage remain unaware).
In his asides, Macbeth reveals that he is beginning to believe the prophecy of the three witches, because he is already thane of Glamis and now is thane of Cawdor, so the kingship may not be far out of reach: "Glamis, and thane of Cawdor!/The greatest is behind."
Macbeth goes on to intimate that the only way he could become king would be to murder the present king and take the throne by force. He cautions himself that he is only imagining murder, not actually planning to commit it:
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.
Macbeth dismisses his own concerns by pointing out that he has become thane of Cawdor by chance, so perhaps he will attain the kingship by chance as well.