Chapter XVIII. Comparison of a soul and a millstone. And this movement of the heart is not unsuitably illustrated by the comparison of a mill wheel, which the headlong rush of water whirls round, with revolving impetus, and which can never stop its work so long as it is driven round by the action of the water: but it is in the power of the man who directs it, to decide whether he will have wheat or barley or darnel ground by it. That certainly must be crushed by it which is put into it by the man who has charge of that business. So then the mind also through the trials of the present life is driven about by the torrents of temptations pouring in upon it from all sides, and cannot be free from the flow of thoughts: but the character of the thoughts which it should either throw off or admit for itself, it will provide by the efforts of its own earnestness and diligence: for if, as we said, we constantly recur to meditation on the Holy Scriptures and raise our memory towards the recollection of spiritual things and the desire of perfection and the hope of future bliss, spiritual thoughts are sure to rise from this, and cause the mind to dwell on those things on which we have been meditating. But if we are overcome by sloth or carelessness and spend our time in idle gossip, or are entangled in the cares of this world and unnecessary anxieties, the result will be that a sort of species of tares will spring up, and afford an injurious occupation for our hearts, and as our Lord and Saviour says, wherever the treasure of our works or purpose may be, there also our heart is sure to continue. 1131 1131     Cf. S. Matt. vi. 21 .