Personal space, an upated form of Edward T. Hall's 1966 proxemics, is the region surrounding each person, or that area which a person considers their domain or territory. Often if entered by another being without this being desired, it makes them feel uncomfortable. The amount of space a being (person, plant, animal) needs falls into two categories, immediate individual physical space (determined by imagined boundaries), and the space an individual considers theirs to live in (often called habitat). These are dependent on many things, such as growth needs, habits, courtships, etc. Hall's spacing models, to note, were themselves based on Heini Hediger's 1955 psychological studies of zoo animals.