Watch over your desires, such as would dull and blunt the edge of the spirit. As iron drives out iron, so one desire drives out another. If we are taken with other things, Christ loses His sweetness and relish. Vain worldly desires extinguish those that are spiritual and heavenly: they lose their fervour when prostituted to base objects; your prayers are more flat and cold, for your desires are manifested by prayer and industry.
Now your desires will flag and abate when you let out your hearts to the world; therefore you must watch lest the carnal savour and carnal minding increase upon you, for then the spiritual minding is quite hindered, impeached, and interrupted: “For they that after the flesh do mind the things of the flesh, but they that are after the spirit the things of the spirit” (Rom 8:5). When outward things would steal away your hearts and affections from God, remember Him who is more desirable.
Adapted from The Complete Works of Thomas Manton, 6:433.