Two words: free will.
Three more words: does not exist
Nisbett, Richard, & Wilson, Timothy. (1977). Telling more than we can know: Verbal reports on mental processes. Psychological Review, 84, 231-259.
Shows that even after we have made a supposedly free choice, we don't know what were the real determinants of our behavior. We report plausible, but baseless explanations.
Libet, Benjamin (1985). "Unconscious Cerebral Initiative and the Role of Conscious Will in Voluntary Action". The Behavioral and Brain Sciences 8: 529â"566.
Shows that unconscious neural processes that launch behavior precede the decision to do the behavior. We feel like we made a free choice to act after the act has already begun.
An experimental assessment of facilitated communication. Wheeler, Douglas L.; Jacobson, John W.; Paglieri, Raymond A.; Schwartz, Allen A. Mental Retardation, Vol 31(1), Feb 1993, 49-59.
Shows that even when we are certain we are not doing something we adamantly say we would not do, we unconsciously choose...but not freely...to do it.