Meanings of posit the existence in English

Some of the blacks of my acquaintance are ardent believers in ghosts and do posit the existence of personal "debils-debils."

I felt happily free to posit the existence of any old exotic psycho-flora or cosmo-fauna and, if challenged, retreat to my hermeneutic castle.

The date is the birthday of Percival Lowell, the astronomer that posited the existence of a "Planet X" beyond the orbit of Neptune.

But now, Bern is betting big on a once sidelined theory called supergravity, which posits the existence of new gravity-related particles that mirror gravitons' effects.

"As, however, there is no necessity whatever why we should posit the existence of devils, why, then, should they be posited?"