In Act One, scene four, Hamlet joins Horatio and...
At the beginning of William Shakespeare's play, Hamlet, Horatio comes to Hamlet to tell him that Old Hamlet's ghost has appeared on the battlements. Hamlet assures Horatio that he will join him there later, to see the spirit himself.
In Act One, scene four, Hamlet joins Horatio and Marcellus on the battlements at midnight. When Hamlet sees the Ghost, he reacts in several ways.
First, Hamlet approaches the Ghost, determined to speak to it.
...Be thou a spirit of health or goblin damn'd,
Bring with thee airs from heaven or blasts from hell,
Be thy intents wicked or charitable, (45)
Thou comest in such a questionable shape
That I will speak to thee. I'll call thee Hamlet,
King, father, royal Dane. O, answer me!
Hamlet wants to know why the Ghost, looking so like his father, is walking the earth.
When the Ghost wants Hamlet to follow, Horatio and Marcellus try to dissuade him, but Hamlet curiously moves to follow the spirit, still not certain that it is his father's ghost.
It will not speak; then will I follow it.
Do not, my lord!
Why, what should be the fear?(70)
I do not set my life at a pin's fee;
And for my soul, what can it do to that,
Being a thing immortal as itself?
It waves me forth again. I'll follow it.
At this point, Horatio and Marcellus, fearful that the Ghost may be evil, sent from hell to trick Hamlet, try to physically prevent Hamlet from following. Hamlet defies them with his bravery, and furiously threatens the men—committed for his father's sake to hear what the Ghost has to say:
My fate cries out,(90)
And makes each petty artery in this body
As hardy as the Nemean lion's* nerve.
Still am I call'd. Unhand me, gentlemen.
By heaven, I'll make a ghost of him that lets me.
I say, away! Go on. I'll follow thee. (95)
From the beginning of this scene until the end, Hamlet welcomes the possibility that the Ghost may be his father. He is curious. He is committed. And ultimately, he bravely, even defiantly, chooses to follow the Ghost to hear what it has to say. Hamlet explains to his companions that his mind is not only committed to his purpose, but the very arteries in his body hum with the need to follow the specter. Hamlet is firm of purpose and clear of mind.
Hamlet's state of mind in scene four is clear, and he is determined to speak with the Ghost.
*Nemean lion's nerve: "As hardy as the nerve of the lion killed by Hercules"