Doubt can only be removed by action.

What must the English and French think of the language of our philosophers when we Germans do not understand it ourselves?

And I have again observed, my dear friend, in this trifling affair, that misunderstandings and neglect occasion more mischief in the world than even malice and wickedness. At all events, the two latter are of less frequent occurrence.

How can one learn to know oneself? Never by introspection, rather by action.

Always to distrust is an error, as well as always to trust.

How often do I lull my seething blood to rest, for you have never seen anything so unsteady, so uncertain, as this heart.

The world of empirical morality consists for the most part of nothing but ill will and envy.