The brain is intrinsically physical, and subject to thousands of influences including injury, chemicals manufactured within the body, chemicals invading or inserted into the body, and the effects of aging. One has only to visit mental wards, nursing homes, and back alleys to see humans out of touch and apparently in another world. Even primitive man suffered these problems, becoming psychotic with grief or fear or even born psychotic, and discovering to their horror or pleasure that certain foods did more than fill their bellies. Is psychosis and senility intrinsic to man, and does it have a purpose? Yes on both counts, though it often seems a purposeless affliction to those intent on keeping the gears of society running smoothly.
As with fainting, psychosis and senility allows the human animal to disconnect with reality. Catatonic or autistic individuals are, chemically, in a place where they are not feeling anxiety. The world does not exist for them. Delusions serve the same purpose, as the individual can build a world about them that meets their needs. How much different are delusions from the games people play with themselves to make themselves more secure, more attractive, or more valued in their own eyes than they are in the eyes of others. Depression serves a purpose in causing the individual to retract and withdraw from a world that is causing injury. Time to reflect and plot a new course. Senility, outside of the genetic disease which is Alzheimer's, is greatest in those who have the least to live for. Activity and involvement actually prevents senility. Senility blunts the awareness of the aged, so they can reminisce about happier days when they were younger.
Human society sees psychosis as a problem because it is a disruption, but mostly because of the apparent pain the afflicted are in. Is the autistic child not frozen in fear? Is the chronic depressive not missing out on life? Is the paranoid schizophrenic not constantly on the alert and never at rest? What is not taken into the equation is what the world of the individual is like without psychosis - where pain is born quietly but is more intense. Psychosis is noisy and noticeable, where the pre-psychotic individual is usually quiet and well behaved. Then is psychosis not a problem? Of course it is, as it is the signal flag of misery.
Infants born psychotic are thus because the genetic throw of the dice gave them an ultra sensitive nature, and they are doomed to live in the sheltered world they live in. Depressives recover when they change the life situation that is distressing them, sometimes with dramatic swiftness. That so many depressives do not recover but mask their unhappiness with antidepressants does not point to the illness as being intractable so much as it points to the rigidity of society. Schizophrenics react to the same stresses as other humans, but with a stronger and quicker reaction. This is widely recognized in giving schizophrenics a more sheltered environment, but as the press is for society to reabsorb the individual, any respite from the chemical surges is short lived. Anti-psychotic drugs simply mask the raging surges, as though the fire hose were perpetually turned on the blaze so that one can say that the fire is out. The problem here is not so much that a psychosis ensues due to life stresses as that society has rigid expectation for all its citizens.
Does psychosis occur in the animal kingdom, outside of the human animal? Most certainly, a fact that veterinarians will be the first to attest to. Psychotic pets, however, are usually given the life changes that are indicated, and recover. Human society is not so kind to the human animal, who is generally drugged and told to struggle on, in place.