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.