Search me, O God, and know my heart: try me, and know my thoughts: And see if there be any wicked way in me, and lead me in the way everlasting.
(Psa 139:23-24 KJV)
“The heart of man is full of sin and corruption, and that corruption is of an exceedingly darkening, blinding nature. Sin always carries a degree of darkness with it; and the more it prevails, the more it darkens and deludes the mind.- It is from hence that the knowing whether there be any wicked way in us is a difficult thing. The difficulty is not at all for want of light without us, not at all because the word of God is not plain, or the rules not clear; but it is because of the darkness within us. The light shines clear enough around us, but the fault is in our eyes; they are darkened and blinded by a pernicious distemper.
Sin is of a deceitful nature, because, so far as it prevails, so far it gains the inclination and will, and that sways and biasses the judgement.”
-Jonathan Edwards, from Christian Cautions