In the Aristotelian conception of the good life reason is an important factor in the achievement of all the virtues. It is an essential element in the doctrine of the golden mean which tells us that a virtue is the point which is midway between the extremes of excess and deficiency. The determination of this point will vary with individuals and their respective circumstances for it is not the mathematical mean but the organic mean as determined by "reason" that prescribes what each individual ought to do. This is an important point in Aristotle's ethics for quite in contrast with what some moralists of the present day are advocating, he does not believe that the nature of goodness is purely a matter of satisfying one's desires. To be sure he recognizes that desires are an important element in the good life but unless these desires are given guidance and direction by the reason they may hinder rather than promote the realization of the good life.
In view of the fact that reason is the guiding element in all of the virtues it may seem strange that an entire book of the Ethics should be devoted to the intellectual virtues thus implying a distinction between the intellectual virtues and the moral virtues. There is a sound basis for this distinction although it does not mean that the two types of virtues are entirely separate or that either one functions independently of the other. The distinction is primarily that of means and ends. In the moral virtues the emphasis is placed on the proper control of one's appetites and desires. This must be done as a means toward the achievement of some larger and more inclusive end. Temperance thus becomes a means toward the acquisition of good health. Courage which always involves a risk is a necessary means for the further development of one's capacities and powers. But that which is a means must always be a means for something and somewhere along the line there must be a final end or goal which has value in itself. This is what Aristotle finds in the development of man's intellectual capacities. Wisdom is not only a virtue but it stands highest among all the virtues. It is the realization of a capacity which distinguishes man from the lower animals and gives to him a kind of kinship with the gods. The fact that wisdom is an end in itself does not mean that it is useless for anything else. It can be used to direct life's activities but it also has a positive value in addition to this use, for it is in contemplation that man finds his greatest happiness and the fulfillment of that which is unique in his nature.
It is through the development of the intellect that man acquires knowledge of the sciences. Scientific knowledge includes two elements. One of these has to do with the unchanging principles or laws of nature and the other one deals with the changing or the contingent factors that are present in the processes of the world. It is through sensation that we are made aware of that which changes from time to time but it is only through the intellect that we gain knowledge of the permanent or unchanging principles which enable us to make predictions and in the light of these to organize the world of our experiences. That which we obtain through the intellect enables us to make application of our scientific knowledge both in the realm of the arts and in the pursuit of the various vocations. In the field of ethics the same as in the area of the natural sciences it is necessary to have principles and to know how to apply them to particular cases. It is through the use of reason that both of these may be accomplished. The field of ethics is however somewhat different from that of the natural sciences since its aim is to know what one ought to do rather than to describe things as they actually exist. In the sciences one may verify conclusions by making predictions about what will happen under specified conditions and then observing to see whether these predictions have been fulfilled. One cannot do this in the field of ethics for no amount of information about what is can ever tell one what ought to be. Nevertheless, it is the function of ethics to discover the right principles of conduct and this involves knowledge of the final end or goal of life as well as the appropriate means for reaching it.
In matters of this kind there is no substitute for sound judgment or what we are accustomed to speak of as good common sense. Plato had taught that knowledge of the good was the most important quest that could ever occupy the mind of man and Aristotle appears to be in full accord with this view. But how is this knowledge to be obtained? Obviously it cannot be observed directly and neither is there any supreme authority from which it can be handed down to us. It is through a kind of intuitive insight that the mind grasps the principles of conduct that may point the way toward the good life. This does not mean that the ideas which flash into a person's mind are for that reason infallible. There are false intuitions as well as correct ones and it is the function of the reason to distinguish between them. Correct intuitions must be consistent with themselves and in harmony with all the known facts. Further than this they must provide an intelligible and meaningful interpretation of one's experiences. Intuitions of this type do not as a rule occur to the ignorant or uninformed person or if they do he probably would not recognize them. For this reason one should look to those who are highly trained in the field for guidance and for fruitful suggestion. But their views need also to be subjected to rational criticism and accepted only insofar as they appear to meet the criteria for sound judgment. Obviously one cannot have the same degree of certainty in the field of ethics that he may have in the formal and in the natural sciences. Even so, the decision is not left to blind chance for it is always possible to select the course of action which in the light of the information he may have appears to be the most reasonable.