An excerpt from John Piper’s foreword to the new edition of Thomas Chalmer’s classic, The Expulsive Power of a New Affection, published in the Crossway Short Classics series.

I recall once being asked a trick question: If you had access to all the latest machinery in a sophisticated science lab, what would be the most effective way to get all the air out of a glass beaker? One ponders the possible ways to suck the air out and create a vacuum. Eventually, the answer is given: fill it with water.

That is the point of Chalmers’s famous message. It is intended as an illumination of 1 John 2:15:

Do not love the world or the things in the world. If anyone loves the world, the love of the Father is not in him.

Chalmers poses for himself the question: How shall the human heart be freed from its love for the world? (How shall the air of world-love be removed from the soul-beaker?) This “love” is not a duty one performs. It is a delight one prefers. It is an affection before it is a commitment.

He says there are two ways one might seek to remove this controlling affection from the heart. One is to show that the world is not worthy of our affection and will let us down in the end. (This argument corresponds to using a pump to suck the air out of the beaker.)

The other is to show that God is vastly more worthy of the heart’s attachment, thus awakening a new and stronger affection that displaces the former affection for the world. (This corresponds to pouring water into the beaker to displace the air.) Hence “the expulsive power of a new affection.” Chalmers himself states his purpose,

My purpose is to show that from the constitution of our nature, the former method is altogether incompetent and ineffectual and that the latter method will alone suffice for the rescue and recovery of the heart from the wrong affection that domineers over it.

You can read the whole foreword here, and purchase the book here.