 It is considerably less clear what these objects are and how it is that experience actualizes the relevant potentialities in the soul. Aristotle describes a series of stages of cognition. First is what is common to all animals: perception of what is present. Next is memory, which he regards as a retention of a sensation: only some animals have this capacity. Even fewer have the next capacity, the capacity to form a single experience (empeiria) from many repetitions of the same memory. Finally, many experiences repeated give rise to knowledge of a single universal (katholou). This last capacity is present only in humans.