The frontal lobes are the executive center of the brain. The wisdom of nature is reflected in the fact the frontal lobes contain a high concentration of receptors that are designed to respond to endorphins. This means that when the executive center of the brain is fully engaged, we are more likely to be filled with good feelings. Hence, how we feel is a measure of the quality of our thinking. If your beliefs (mental models) make you weary, depend upon it, you are not fully utilizing the full benefits of your design. Self-knowledge is the path to full engagement and our feelings signal us when we are headed in the wrong direction.