“Beside Oneself”: An expression used since 1400s, it is an inadvertent reference to multiple personality, for how else could people be beside themselves?

There is an intuitive recognition of multiple personality in the language of everyday life.

When people are “in their right mind” (which implies they have other, wrong minds), they have “got it together” (which implies that it is possible to have it separately or dissociated).

When people are upset, not in their right mind, and don’t have it together, they may feel beside themselves (1, 2, 3).

People appear beside themselves, and overtly manifest their multiple personality, only when something has upset them. This is a recognition of the connection between trauma and multiple personality.

1. http://idioms.thefreedictionary.com/beside+oneself

2. http://www.todayifoundout.com/index.php/2015/01/origin-beside/

3. http://www.grammarphobia.com/blog/2015/03/beside-onerself.html