"Because the end itself determines the well-functioning of the human person. The disposition has, as a part of its very nature, a tendency towards a specific end. This end, when realized, contributes to the well-being of the individual. This is the crux of natural law theory. Nature has `determined', as it were, the ends which lead to the well-being of the individuals of the natural kind."
A. Lisska, Aquinas's Theory of Natural Law: An Analytic Reconstruction, p. 107