Such an accidental stability of structure is, in this moving world, a sufficient principle of permanence and individuality. The same mechanical principles, in more complex applications, insure the persistence of animal forms and prevent any permanent deviation from them. What is called the principle of self-preservation, and the final causes and substantial forms of the Aristotelian philosophy, are descriptions of the result of this operation. The tendency of everything to maintain and propagate its nature is simply the inertia of a stable juxtaposition of elements, which are not enough disturbed by ordinary accidents to lose their equilibrium; while the incidence of a too great disturbance causes that disruption we call death, or that variation of type, which, on account of its incapacity to establish itself permanently, we call abnormal. Nature thus organizes herself into recognizable species; and the aesthetic eye, studying her forms, tends, as we have already shown, to bring the type within even narrower limits than do the external exigencies of life. _The relation of utility to beauty._ Sec. 39. This natural harmony between utility and beauty, when its origin is not understood, is of course the subject of much perplexed and perplexing theory. Sometimes we are told that utility is itself the essence of beauty, that is, that our consciousness of the practical advantages of certain forms is the ground of our aesthetic admiration of them. The horse's legs are said to be beautiful because they are fit to run, the eye because it is made to see, the house because it is convenient to live in. An amusing application -- which might pass for a _reductio ad absurdum,_ -- of this dense theory is put by Xenophon into the mouth of Socrates. Comparing himself with a youth present at the same banquet, who was about to receive the prize of beauty, Socrates declares himself more beautiful and more worthy of the crown. For utility makes beauty, and eyes bulging out from the head like his are the most advantageous for seeing; nostrils wide and open to the air, like his, most appropriate for smell; and a mouth large and voluminous, like his, best fitted for both eating and kissing.[11]