If a past custom no longer realizes the ideal human relationship amongst people, then the old customs must be changed. Li assumes that the moral ideal is fully known and represented by past practice, but jen requires that we never claim full knowledge of the ideals towards which humanity should journey, that we must adapt the ideal to new situations. The key concept that summarizes this flexible adaptation of the ideal is known as the Golden Mean in Aristotle's philosophy and as Chung Yung in Confucianism. The Golden Mean as the key to virtue is inherent in the Greek philosophers, Plato and Aristotle, who urged a moderate fulfilment of all basic human appetites so that a person would in one's moral character have a wise internal balance and beauty just as a Greek statue would have beautiful external proportions. Going to the extreme in any one appetite would destroy a person's moral character just as going to an extreme in the design of an arm of a statue would make the statue grotesque.
Moderation is the key.
Aristotle exemplifies this by considering what is the proper amount of food for an individual to eat. There is a great deal of difference for someone weighing 120 lbs. Yes, they both need a balanced diet, but one needs fewer calories than the other. Either one could eat too few calories or too many calories. Each must eat a mean, a middle, between too little and too much, but the mean is different for each person.