Apart from using our eyes to see and our ears to hear, we regularly and effortlessly perform a number of complex perceptual operations that cannot be explained in terms of the five senses taken individually. Such operations include, for example, perceiving that the same object is white and sweet, noticing the difference between white and sweet, or knowing that one's senses are active. Observing that lower animals must be able to perform such operations, and being unprepared to ascribe any share in rationality to them, Aristotle explained such operations with reference to a higher-order perceptual capacity which unites and monitors the five senses.