Reason is impaired by passions

This does the poor philosopher reason when he undertakes to reason at moments when a tumultuous passion leads the divine faculties of his soul astray. To reason rightly one must be neither in love nor in anger; for those two passions reduce as to the level of animals; and unfortunately we are never so much inclined to reason as when we are agitated by one or the other of them.

Giacomo Casanova - History of my life (2, 1)