It cannot be said that man's happiness should arise from any kind of life, for even plants have life. But happiness is sought as a good characteristic of man since it is called a human good. Likewise, happiness must be different from the life of nutrition or growth, which even vegetables possess. From this we take it that happiness does not consist in health, beauty, strength, or great stature, for all these things result from activities of vegetative life.
On the step above the life of mere nutrition and growth is the life of sense experience. Again, this is not proper to man but is possessed by horses, oxen, and other animals. In this kind of life, then, happiness does not consist. So we conclude that human happiness is not found in any form of sense perception or pleasure.
Beyond the life of assimilation and of sense experience there remains only the life that functions according to reason.
[Thomas Aquinas, Commentary on Aristotle's Nicomachean Ethics, Litzinger, tr., Dumb Ox Books (Notre Dame, IN: 1964) p. 42.]
