Mesillas Yesharim 14 (Chapter 3)
The way to overcome one's Yetzer Hara is to weigh the outcome of each action, good or bad (על כן יאמרו המושלים בואו חשבון - see בבא בתרא עח). The greatest challenge in life is living under the influence of the Yetzer Hara which makes it hard to make proper decisions. The Yetzer Hara tries to obscure our judgment by preventing us from seeing clearly what is right and what is wrong. How? By trying to keep us living in the darkness.
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