A book is a mirror: if an ape looks into it, an apostle is unlikely to look out.

Georg C. Lichtenberg (1742-1799) German physicist, writer

Aphorisms, Notebook F, #17 (1776-79) [tr. Hollingdale (1990)]
    (Source)

This is nearly mirrored by Notebook E, # 49 (1775-76), "A book is a mirror: if an ape looks into it an apostle is hardly likely to look out."

Alternate translations:

A book is a mirror: when a monkey looks in, no apostle can look out.
[tr. Mautner and Hatfield (1959)]

A book is a mirror: if an ape looks into it, an apostle is unlikely to look out.
[tr. Tester (2012)]