This phrase is from Horace's Epistula II, line 40:
Dimidium facti qui coepit habet: sapere aude
"He who has begun is half done: dare to know!"
Sapere aude was the motto of the Society of the Friends of Truth, a major mover and shaker in the German Enlightenment, hence Kant's declaring it the "motto of the enlightenment" in his essay, "An Answer to the question: What is Enlightenment?", which first appeared in the Berlinische Monatsschrift in December, 1784.