Singer holds that a being's interests should always be weighed according to that being's concrete properties. He favors a 'journey' model of life, which measures the wrongness of taking a life by the degree to which doing so frustrates a life journey's goals.[clarification needed] The journey model is tolerant of some frustrated desire and explains why persons who have embarked on their journeys are not replaceable. Only a personal interest in continuing to live brings the journey model into play. This model also explains the priority that Singer attaches to interests over trivial desires and pleasures.