One key to understanding living organisms, from those that are made up of one cell to those that are made up of billions of cells, is the definition of their boundary, the separation between what is in and what is out. The structure of the organism is inside the boundary and the life of the organism is defined by the maintenance of internal states with in boundary. Singular individuality depends on the boundary.

– Antonio Demasio, The Feenling of What Happens.