Many arguments have been put forward on both sides of the debate, but here I will focus on the experiment carried out on a patient by Benjamin Libet.
Libet, while observing the patient’s brain, told him to move his hand whenever he wished. Libet found that the brain registered a desire to move the hand, even before the man himself was aware of this desire.
This suggests that we may act impulsively, without actually making a decision. It only seems to us afterwards that we did something from our own free will, since we rationalize our action after it happens.