For Frankfurt the fundamental dimension of our humanity is our ability to reflect on our own mental life. We can have thoughts about thoughts, feelings about feelings, etc. It is in the process of finding rectitude between these layers of experience and reflection that we work out who we are and how we live our lives. Thus, for Frankfurt, concepts such as externality (things that occur within our mental histories but are not necessarily part of our identity), wholeheartedness, identity, and will are central to understanding what kind of beings we are. Frankfurt does not seek to define humans from the outside but, rather, from the inside by route of what we care about and the extent of our efforts.