Conscience adjusts to whatever a person repeatedly does, so it must be trained toward what is truly right rather than left to feelings. Sin tarnishes the mind so that a person no longer looks at life the same way, and Paul says that to the defiled and unbelieving, even mind and conscience are defiled (Titus 1:15). A conscience can err in either direction. A man who had been a lifelong vegetarian accepted that eating meat is permissible, yet the first time he tried it he became so overwrought that he vomited, because his mind had not yet adjusted to the truth and reacted as if the act were evil. In the same way, when a person commits a sin, he feels ill at ease at first, but if he continues, his conscience gradually adjusts. Each sin makes the next easier, until the conduct becomes acceptable and he sins without a qualm. Sin's deceitfulness hardens the heart, producing insensitivity of conscience, inflexible thinking, and finally an inability to repent. The conscience is therefore educated by God's law together with the Holy Spirit, which lift the lid from self-deceived respectability and reveal what we and sin are like underneath. Understanding how the Bible perceives sin, and checking for it often within ourselves, keeps the conscience sensitive.