How can we say that some outside things are natural and others unnatural? Think of it this way. If we were cut off from everyone else, we might say different things. Take a foot, for example. I might say it's natural for a foot to stay clean. But if you think of it as a foot that belongs to a body, then it should step in mud and walk on thorns when needed. Sometimes it should even be cut off to save the whole body. Otherwise, it's not really doing its job as a foot. We should think about ourselves the same way. What are you? A human being. If you think of yourself as separate from other people, then yes — it seems natural to live to old age, be rich, and stay healthy.
How then is it said that some external things are according to nature and others contrary to nature? It is said as it might be said if we were separated from union (or society): for to the foot I shall say that it is according to nature for it to be clean; but if you take it as a foot and as a thing not detached (independent), it will befit it both to step into the mud and tread on thorns, and sometimes to be cut off for the good of the whole body; otherwise it is no longer a foot. We should think in some such way about ourselves also. What are you? A man. If you consider yourself as detached from other men, it is according to nature to live to old age, to be rich, to be healthy.