Cheerfulness

Consider someone who has every reason to be happy but who continually sees enacted before him tragedies full of disastrous events, and who spends all his time brooding on sad and pitiful objects. Suppose that he knows that these are imaginary fables: they draw tears from his eyes and move his imagination, but don’t touch his intellect at all. I think that this by itself would be enough gradually to constrict his heart and make him sigh in
such a way that the circulation of his blood would be clogged and slowed down. The bigger parts of his blood, sticking together, could easily block the spleen, getting caught in it and stopping in its pores; while the more finely divided parts, being continually agitated, could affect his lungs and
cause a cough which in time might be very dangerous.

•Now consider someone who has countless genuine reasons for distress but who takes such trouble to direct his imagination that he never thinks about them except when some practical necessity forces him to, and who spends the rest of his time
thinking about things that can give him contentment and joy.
This will greatly help him by enabling him to make sounder judgments about the things that matter to him, because he’ll look on them without passion. Furthermore, I am sure that this by itself could restore him to health, even if his spleen
and lungs were already in a poor condition because of the bad condition of the blood caused by sadness.

I know that everything I write here is better known to you than to me, and that what’s difficult in this matter is not the theory but the practice. Still, the great favour that you do me in showing that you aren’t averse to hearing my views makes me take the liberty of writing them down just as they are,
and of adding this: The remedy I have just suggested cured an illness of mine that was very like yours and perhaps even more dangerous. I was born of a mother who, a few days [actually, 14 months] after my birth, died from a disease of the
lungs caused by distress. From her I inherited a dry cough and a pale colour which stayed with me until I was more than twenty, so that all the doctors who saw me predicted that I would die young. But I have always tended to look at things from the most favourable angle and to make my chief happiness depend upon myself alone; and I believe that this
tendency caused the indisposition gradually to disappear completely—the indisposition that was almost part of my nature

Descartes ( from a letter to Princess Elisabeth)