“Look within. Within is the fountain of the good, and it will ever bubble up, if thou wilt ever dig.”

Canto III, stanza 16 (Coronach, stanza 3).
The Lady of the Lake http://www.gutenberg.org/etext/3011 (1810)

Source: Meditations

Tabulae Votivae (Votive Tablets) (1796), "The Key"; tr. Edgar Alfred Bowring, The Poems of Schiller, Complete (1851)
Variant translation:[citation needed]
If you want to know yourself,
Just look how others do it;
If you want to understand others,
Look into your own heart

Tractatus VII, 8 http://www.newadvent.org/fathers/170207.htm
Latin: "dilige et quod vis fac."; falsely often: "ama et fac quod vis."
Translation by Professor Joseph Fletcher: Love and then what you will, do.
In epistolam Ioannis ad Parthos

X, 31
Meditations (c. 121–180 AD), Book X
Context: Continuously thou wilt look at human things as smoke and nothing at all; especially if thou reflectest at the same time, that what has once changed will never exist again in the infinite duration of time. But thou, in what a brief space of time is thy existence? And why art thou not content to pass through this short time in an orderly way?

Preface
The Way to Christ (1622)
Context: If thou wilt use these Words aright, and art in good Earnest, thou shalt certainly find the Benefit thereof. But I desire thou mayest be warned, if thou art not in Earnest, not to meddle with the dear Names of God, in and by which the most High Holiness is invoked, moved, and powerfully desired, lest they kindle the Anger of God in thy Soul. For we must not abuse the Holy Names of God. This little Book is only for those that would fain repent, and are in a Desire to begin. Such will find what Manner of Words therein, and whence they are born. Be you herewith commended to the Eternal Goodness and Mercy of God.

Source: Diary of a Drug Fiend

The Obedience of A Christian Man (1528)