Love Letters of Great Men.

David Hume was a philosopher, economist and historian. He lived an exemplary scholarly life until 1763, when he visited Paris for the first time and stayed for more than 2 years. He seems during his period to have suffered some kind of mid-life crisis; celebrated in the salons of the great Parisian ladies, he became particularly enamored of one Madame de Boufflers, already the mistress of the prince de Conti. But the lady was a great deal more experienced that the philosopher in such flirtations and the smitten Hume grew more and more confused. When her husband died it became clear that she hoped to marry the prince, and Hume ultimately found himself in the rather unsatisfactory role of confidant to both.
Here is a letter to her...

To Madame de Boufflers, 3 April 1766.

It is impossible for me dear madame, to express the difficulty which i have to bear your absence, and the continual want which i feel of your society. I had accustomed myself, of a long time, to think of you as a friend from whom i was never to be separated during any considerable time; and i had flattered myself that we were fitted to pass our lives in intimacy and cordiality with each other. Age and natural equality of temper were in danger of reducing my heart to too great indifference about everything, it was enlivened by the charms of your conversation, and the vivacity of your character. Your mind more agitated both by unhappy circumstances in your situation and by your natural disposition, could repose itself in the more calm sympathy which you found with me.

But behold! 3 months are elapsed since i left you;and it is impossible for me to assign a time when i can hope to join you. I still return to my wish, that i never left Paris, and that i had kept out of the reach of all other duties, except that which was so sweet, and agreeable, to fulfil, the cultivating your friendship and enjoying your society. Your obliging expressions revive this regret in the strongest degree; especially where you mention the wounds which, though skinned over, still fester at the bottom.

OH! my dear friend, how i dread that it may still be long ere you reach a state of tranquility, in a distress which so little admits of any remedy, and which the natural elevation of your character, instead of putting you above it, makes you feel with great sensibility. I could only wish to administer the temporary consolation, which the presence of a friend never fails to afford... I kiss your hands with all the devotion possible.
Hume.