WIST

Such as are your habitual thoughts, such also will be the character of your mind; for the soul becomes dyed with the color of its thoughts.

Marcus Aurelius (AD 121-180) Roman emperor (161-180), Stoic philosopher
Meditations, Book 5, #16

Alt. trans.:

• "Your mind will be like its habitual thoughts; for the soul becomes dyed with the color of its thoughts."
• "Whatever kind of impressions you receive most often, so too will be your mind, for the soul is dyed with the color of one's impressions." [tr. Needleman & Piazza (2008)]
• "Your manners will depend very much upon the quality of what you frequently think on; for the soul is as it were tinged with the color and complexion of thought." [tr. Collier (1887)]

The last clause is also frequently attributed to William Ralph Inge, who likely used it in an essay.