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 posses. 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. This life is proper to man, for he receives his specific classification from the fact that he is rational. Now the rational has two parts. One is rational by participation insofar as it is obedient to and is regulated by reason. The other is rational by nature as it can of itself reason and understand. The rational by nature is more properly called rational because a thing possessed intrinsically is always more proper than a thing received from another. Since, therefore, happiness is the most proper good of man, it more likely consists in the rational by nature than in the rational by participation. From this we can see that happiness will more properly be found in the life of thought than in a life of activity, and in an act of reason or intellect than in an act of the appetitive power controlled by reason.
Thomas Aquinas, Commentary on Aristotle's Nicomachean Ethics, Book I, Lecture 10.