Paul Ludwig Carl Heinrich Rée (21 November 1849 – 28 October 1901) was a German author, physician, philosopher, and friend of Friedrich Nietzsche.

Most of the general judgments of his character and work go back to formulations of Nietzsche and their mutual friend Lou Salomé.

Rée's status as the son of a wealthy businessman and landowner allowed him to study philosophy and law at the University of Leipzig.

Rée conglomerated his diverse studies under the heading of "psychological observations", describing human nature through aphorisms, literary and philosophical exegesis.

Reiterating the conclusions of Psychological Observations, Rée claimed altruism was an innate human drive that over the course of centuries has been strengthened by selection.

The error of free will, Rée claims, lies behind the development of the feeling of justice: The feeling of justice thus arises out of two errors, namely, because the punishments inflicted by authorities and educators appear as acts of retribution, and because people believe in the freedom of the will.Rée rejected metaphysical explanations of good and evil; he thought that the best explanations were those offered by Darwin and Lamarck, who had traced moral phenomena back to their natural causes.

"[2] Rée's friendship with Nietzsche disintegrated in the fall of 1882 due to complications from their mutual involvement with Lou Salomé.