When he(Libet) stimulated their hand's skins, he realized that a certain duration of stimulation, about 500 ms, needed to elicit a certain conscious experience. He called this "neuronal adequacy." The most disputable finding that he observed during his experiments was "a substantial delay before cerebral activities ...achieve neuronal adequacy" when the skin stimulation applied 200 ms after the beginning of a direct stimulation. In that case, subjects felt the skin stimulation before the direct one. Subjects had reported similar experience when those two stimulations were applied simultaneously, suggesting that mental events are ahead of physical ones. Libet concluded that there is "a disassociation between the timings of corresponding 'mental' and 'physical' events," which "raises serious but not insurmountable difficulties for [psychoneural] identity theory" that suggests: All mental events are identical to the brain's processes, and they are also simultaneous. Although Libet's findings are very controversial, they raise serious challenges to those who believe that some of the brain processes involve consciousness causatively.