What are you thinking of

you must train yourself only to think such thoughts that if somebody were suddenly to ask you, ‘What are you thinking of?’ you could reply in all honesty and without hesitation, of this thing or that, and so make it clear at once from your reply that all within you is simple and benevolent, and worthy of a social being who has no thought for pleasure, or luxury in general, or contentiousness of any kind, or envy, or suspicion, or anything else that you would blush to admit if you had it in your mind.

Marcus Aurelius [Meditations III, 4]