In Goethe’s Faust I, a new student comes to ask for advice. Mephisto, dressed as Faust, tells him to take careful notes in lectures, as if the Holy Spirit were dictating. The student answers: Das sollt Ihr mir nicht zweimal sagen! / Ich denke mir, wie viel es nützt / Denn, was man schwarz auf weiß besitzt, / Kann man getrost nach Hause tragen. (You need not tell me twice! I can see how useful it is. For what you have in black and white, you can carry home with confidence.) Schwarz auf weiß, in black and white, is still a common phrase for in writing, in a form that counts as proof and leaves no room for doubt.