In Ioannis Evangelium Tractatus XLIX, 12.
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Thus, a man grows up and comes to the age of reason and begins to feel the power of nature’s law which every man bears stamped in his heart: ‘Do unto others as you would have them do unto you’. Does one learn this in books or, rather, doesn’t one read this in nature itself? Does anyone want to be robbed of what is his? Of course not. Here it is then, the law written in the heart; ‘don’t do unto other what you don’t want done to yourself’. Yet, many men violate this law.