The answer is: The heart

Matt 15:19; Mark 7:21

In studying the origins of sin, it’s intriguing to consider that several Gospel verses point to human intentions and desires as the root source. Jesus, in both Matthew 15:19 and Mark 7:21, communicates a powerful message: it's not merely actions that label us 'unrighteous,' but the thoughts and intentions simmering within our hearts. These passages offer a profound insight that our moral battles begin internally long before they manifest externally.

Interestingly, the word "heart" in the biblical context doesn’t just refer to emotions; it embodies the entire will of a person—it represents what shapes our choices and ultimately our actions. For instance, in ancient Hebrew culture, the heart signifies both intellect and emotion, indicating that sin is not confined to intellectual acknowledgment but encompasses feelings and motivations as well.

If we traverse through to the prophetic books, we can reflect on Jeremiah 17:9, where it states that "the heart is deceitful above all things." This doesn’t just reinforce the idea of human imperfection but paints a picture of the challenging inner landscape folks must navigate daily.

Keep in mind, the instruction against sin is not merely to enforce a moral boundary but to draw individuals toward growth and deeper understanding of themselves. This inward reflection can lead to personal transformation, aligning one's actions with kindness and authenticity instead. It’s fascinating how just a few words can shift one's understanding of moral behavior, pointing us towards introspection as a skill we all can develop!