“A book is a mirror: if an ape looks into it an apostle is hardly likely to look out.”
E 49
Variant translations of first portion: A book is a mirror: If an ape peers into it, you can't expect an apostle to look out.
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.
Aphorisms (1765-1799), Notebook E (1775 - 1776)
Context: 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.
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Georg Christoph Lichtenberg 137
German scientist, satirist 1742–1799Related quotes

The Duty of Owning Books (1859)
Context: Books are the windows through which the soul looks out. A house without books is like a room without windows. No man has a right to bring up his children without surrounding them with books, if he has the means to buy them. It is a wrong to his family. He cheats them! Children learn to read by being in the presence of books. The love of knowledge comes with reading and grows upon it.
Source: When Gravity Fails (1986), Chapter 2 (p. 13).

The Jimmy Kimmel Show (4th September 2003)