I seem to remember there was a medieval philosopher--maybe Russell mentions him

I seem to remember there was a medieval philosopher--maybe Russell mentions him

I seem to remember there was a medieval philosopher--maybe Russell mentions him in his History of Western Philosophy--who talked about peer influence or what a social scientist today might call regression toward the mean. He advised that, to live a saintly life, one should surround oneself with saintly people. Who was he? Did he write anything available today? Any pithy quotes?

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