Mar 7:21 For from within, out of the heart of men...

Mar 7:21 For from within, out of the heart of men, proceed evil thoughts, adulteries, fornications, murders,

Alternative: For it is from within, out of the human heart, comes the worthless ideas: cheating, lust, killing...

For Christ, the body is the shell, the outside. What goes into it, doesn't control our humanity. Our humanity comes from within, from our human awareness and feelings.  Christ uses the human heart as symbolic of our emotional connection with the world, the self-awareness of human desires that he separates very clearly from physical animal desires. In Christ's view of the world, our desires are different from our ability to reason and think.

Animals have sex and kill for food, but they don't do it out of human awareness where human desire drives human decision-making. Animals are not driven by the self-awareness of the human heart. This means that they cannot make misjudgments like humans can. He starts with the vices related to sex and killing to make this contrast.

"Within" is from esothen, which means "from within" and "inward."

"Heart" is from kardia (kardia), which means "heart" and which we discuss in a larger Greek context here. It is a metaphor for "deep."

"Of men" is from anthrôpos (anthropos), which "man," and, in plural, "mankind." It also means "humanity" and that which is human and opposed to that which is animal or inanimate.

"Evil" is from kakos (kakos), which means "bad," "mean," "base," "ugly," "ill-born," "evil," "worthless," "sorry," "pernicious," and "ill."

"Thoughts" is dialogismos , (dialogismos)which means "debate," "argument," and "discussion." It can also mean "balancing of accounts," "calculation," and it is the term for a "circuit court." It isn't usually translated as "thought."

"Proceed" is from ekporeuô (ekporeuomai), which means "to make to go out," "to fetch out," and "to march out."

"Adultery" is moicheia (moicheia) which means "adultery."

"Fornication" is porneia, (porneia) which means "prostitution" for a woman and "fornication" for a man.

"Murder" is phonos (phonos), which means "murder," "slaughter," "homicide," "death as a punishment," and "killing."