From Charles Spurgeon's John Ploughman

Some will say they cannot help having bad thoughts; that may be, but the question is, do they hate them or not? Vain thoughts will knock at the door, but we must not open to them.

Though sinful thoughts rise, they must not reign. He who turns a morsel over and over in his mouth, does so because he likes the flavor, and he who meditates upon evil, loves it, and is ripe to commit it. Snails leave their slime behind them, and so do vain thoughts. Good thoughts are blessed guests, and should be heartily welcomed, well Jed, and much sought after.

Like rose leaves, they give out a sweet smell if laid up in the jar of memory. They cannot be too much cultivated; they are a crop which enriches the soil.