What does it mean to watch your thoughts?

Behold the growth of passion! 

It is but a passing thought perchance, moving the heart. 

Whence is it—who can tell? 

Whence is the sudden cloud darkening the fair heaven? and whence the electric spark? 

Your mind conceives and your heart unless you guard it will nurse the awful birth. 

The fiery influence shoots through your being. 

Your nerves tremble, your blood is aflame. 

And though quiet may be restored, there is that within you, which at any moment may course through your veins afresh. 

For remember, if you had an ocean of the red stream of life, one drop of poison might vitiate it. 

Alas, it is more than a drop—the tempting thought has grown to a power of evil possessing you—a nature within your nature—wild, lawless and leading you captive. 

Sin has taken root in your soul, innocent though it found you. 

How far it may take you God alone can tell.

Watch over your thoughts then, lest they ruin your soul! 

Watch, I say, and stifle sin in its birth. 

It may be a small thing at first, but how awful is the growth, suffusing body and soul with poison, doubly dangerous for its seeming sweetness! 

Has it seized your heart—ah, fly to the Physician.

Letters from Hell