A book is a mirror: if an ape looks into it an apostle is hardly likely to look out. We have no words for speaking of wisdom to the stupid. He who understands the wise is wise already.

A book is a mirror: If an ass peers into it, you can't expect an apostle to look out. — this has actually been the most commonly cited form, but it is based on either a loose non-literal translation or a mistranslation of the German original: Ein Buch ist Spiegel, aus dem kein Apostel herausgucken kann, wenn ein Affe hineinguckt.

s of first portion: A book is a mirror: If an ape peers into it, you can't expect an apostle to look out.