According to Aristotle in "On the Soul", memory is the ability to hold a perceived experience in the mind and to distinguish between the internal "appearance" and an occurrence in the past. In other words, a memory is a mental picture () that can be recovered. Aristotle believed an impression is left on a semi-fluid bodily organ that undergoes several changes in order to make a memory. A memory occurs when stimuli such as sights or sounds are so complex that the nervous system cannot receive all the impressions at once. These changes are the same as those involved in the operations of sensation, Aristotelian , and thinking.