As early as 1926, Fritz Jahr suggested the broadening of Kant’s Categorical Imperative onto all living beings. And while, at that time, Jahr’s animal ethics could have been built upon Ignaz Bregenzer and other scientifically accepted sources, Jahr’s ideas on plant ethics must have relied only on more poetical and philosophical conjectures, like the ones by Richard Wagner, Hans Christian Andersen, or Eduard von Hartmann. Today, we have accummulated certain knowledge about plant physiology, proving the complexity of plant cognition and feeling. Ten years ago, the “Rheinauer Theses on the Rights of Plants” once again provoked discussion, eventually supported by Monica Gagliano, Stefano Mancuso, and other biologists advocating the redefinition of human relation toward plants. In the present paper, we intend to review those arguments, but also to examine whether our ethics should be based upon our knowledge only.