Psychologists often discuss the difference between “temperament” and “personality.” Temperament refers to inborn, biologically based behavioral and emotional patterns that are observable in infancy and early childhood; personality is the complex brew that emerges after cultural influence and personal experience are thrown into the mix. Some say that temperament is the foundation, and personality is the building.

This book is about the melancholic direction, which I call the “bittersweet”: a tendency to states of longing, poignancy, and sorrow; an acute awareness of passing time; and a curiously piercing joy at the beauty of the world. The bittersweet is also about the recognition that light and dark, birth and death — bitter and sweet — are forever paired. “Days of honey, days of onion,” as an Arabic proverb puts it.

According to Free Trait Theory, we are born and culturally endowed with certain personality traits — introversion, for example — but we can and do act out of character in the service of “core personal projects.”
In other words, introverts are capable of acting like extroverts for the sake of work they consider important, people they love, or anything they value highly. Free Trait Theory explains why an introvert might throw his extroverted wife a surprise party or join the PTA at his daughter’s school. It explains how it’s possible for an extroverted scientist to behave with reserve in her laboratory, for an agreeable person to act hard-nosed during a business negotiation, and for a cantankerous uncle to treat his niece tenderly when he takes her out for ice cream.