The Adornment of the Spiritual Marriage, by Jan van Ruysbroeck, , at sacred-texts.com
Now consider in a similitude, how this inward exercise should be performed. When the natural fire has by its heat and power stirred water, or some other liquid, until it bubbles up; then this is its highest achievement. Then the water boils up and falls down to the bottom, and is then stirred again to the same activity by the power of the fire: so that the water is incessantly bubbling up, and the fire incessantly stirring it.
And so likewise works the inward fire of the Holy Ghost. It stirs and goads and drives the heart and all the powers of the soul until they boil; that is, until they thank and praise God in the way of which I have told you. And then one falls down to that very ground, where the Spirit of God is burning. So that the fire of love ever burns, and the man's heart ever thanks and praises God with words and with works and yet always abides in lowliness; esteeming that which he should do and would do to be great, and that which he is able to do to be small.