Evagrius believed all forms of temptation could be reduced to a handful of categories. The Greek term for these categories, logosmoi, has been variously translated as tempting thought, evil reasoning, principal fault, passion, and deadly sin. I opt for the term "pattern of impure thought." These patterns are solicitations to engage in a course of ungodly activity. Without scruple, the opponents of godliness (which include Satan, his followers, the world, and our own fallen natures) offer us hollow promises of pleasure and reward in an effort to influence our choices. They seek our consent to their ungodly suggestions.
As the term implies, patterns are sequences or processes. A single thought does not constitute a pattern of thinking. Our spiritual enemies try to entangle us by persuading us to repeatedly make poor choices. They want us in bondage to sinful habits.
Why are there only eight patterns?
Various lists of impure thoughts were already in circulation when Evagrius moved to the desert. He studied the patterns, refined the definitions and carefully developed his list of eight. Over the last 1,600 years, many revisions to the list have been proposed, but nothing has substantially improved its usefulness. Obviously, the list can be tweaked, but it remains amazingly robust.
Why isn't <fill in the blank> a pattern?
Why isn't murder, lying, stealing, cheating, etc. on the list? Because these acts spring from the patterns. Any sin not specifically mentioned in the list is simply a variation or combination of the eight basic patterns.
Because this is a list of thoughts instead of a list of behaviors, motivation is a key consideration. For instance, murder could be a manifestation of anger, greed, pride, or even lust. What motivates someone to lie, steal or cheat? After reviewing the detailed descriptions of the patterns, try throwing various sinful behaviors at the list. You might be surprised to see that, in some way, everything sticks.
For the record, the most popular candidates for addition to the list are envy, jealousy, fear, blasphemy, and heresy.
Is there biblical support for the patterns?
There is not a single passage of Scripture which lists all eight patterns in order. This does not mean the patterns are unscriptural, but that the Bible is not systematically organized. For instance, there is not a single, comprehensive list of God's attributes in Scripture either, yet theologians have come up with several helpful lists on this very subject. It is possible to know God is all-powerful, all-knowing, forgiving, merciful, loving, etc. because of the full testimony of God's Word. Like all systematic doctrine, these truths are constructed from the whole of Scripture.
While the eight patterns do not appear in Scripture as a single list, each one is mentioned in Scripture many times over.