Epictetus, Golden Sayings 56

How is it then that certain external things are said to be Natural, and others contrary to Nature? 

Why, just as it might be said if we stood alone and apart from others. 

A foot, for instance. I will allow it is natural that it should be clean. But if you take it as a foot, and as a thing which does not stand by itself, it will beseem it, if need be, to walk in the mud, to tread on thorns, and sometimes even to be cut off for the benefit of the whole body. Otherwise it is no longer a foot. 

In some such way we should conceive of ourselves also. What are you? A man. Looked at as standing by yourself and separate, it is natural for you in health and wealth long to live. But looked at as a Man, and only as a part of a Whole, it is for that Whole's sake that you should at one time fall sick, at another brave the perils of the sea, know the meaning of want, and perhaps die an early death. 

Why then be discontent? Know that as the foot is no more a foot if detached from the body, so you in like case are no longer a Man? 

For what is a Man? A part of a City. First of the City of Gods and Men. Next, of that which ranks nearest it, a miniature of the Universal City. . . . 

In such a body, in such a world enveloping us, among lives like these, such things must happen to one or another. Your part, then, being here, is to speak of these things as is right, and to order them as befits the matter.