The fourth (and last) volume of discourses, or sermons, on several scriptures

As the breathing upon a Glass sullies it, and dims the Representation of the Face that looks into it; so the breathing of evil Cogitations upon Conscience, the Glass of the Soul, leaves a Mist and Cloud upon it, that it can but dimly and darkly represent to us our true State.

2. Watch diligently as the Heart it self, so all the Approaches unto the Heart. ¹The Approaches to the Heart are like your Roads to a great City, which are full of Passengers, and usually full of Dirt also: And these are the Senses by which and through which Objects are continually travelling to the Heart, and carry with them a World of Wickedness. These are Sluices, which instead of letting in pleasant Streams to refresh, ⟨p. 382⟩commonly they let in nothing but Mud, which pollutes the Soul. There is no actual Filthiness in the Hearts of any but what enters in by these Inlets; through these the Devil casts in abundance of Filth, stirs up and increaseth indwelling Lust, and, by sinful Objects that the Senses convey to the Soul, dungs that Ground that is of it self but too too fruitful. Thus the Devil makes use of the Ear, through it he blows up the Bladder of Pride, by the Breath of popular Applause and Praise. And thus he makes use of the lascivious Eye as a burning Glass to set the Heart on Fire; and so also he makes use of the other Senses as Sinks of Luxury and Intemperance. Now if you would keep your Consciences clear and undefiled, set a strict Guard and narrow Watch upon all these Passages to your Hearts; critically examine every Thing that goes in, and every Thing that comes out by these Gates; arrest whatever cannot produce its Pass and Warrant from the Word of God; keep the same Watch upon these Gates, that God would keep on the Gate of the Heavenly City, the New Jerusalem. It is said,²