Conscia Mens Recti Famae Mendacia Ridet
Google translates this to "The conscious mind smiles [or laughs] at the rumored lies." After digging a little more, I found an oft-quoted Latin proverb -- "A mind conscious of innocence laughs at the lies of rumor." Between the Latin phrase on Dr. Smith's gravestone and the Latin proverb translated to English, there is a slight difference in words: recti versus innocentiae. This is right versus innocence. So maybe the good doctor's Latin phrase translates to English this way:
A mind conscious of right laughs at the lies of rumor.
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