As originally written, the Jean Grey incarnation of the Phoenix was Jean herself, having attained her ultimate potential as a psi, becoming a being of pure energy and reforming herself as Phoenix, only to become slowly corrupted by the manipulation of such foes as Mastermind and Emma Frost; unable to adapt to her enormous power, Jean was driven mad.
In order to return Jean to the fold several years later, this storyline was retconned to reveal the existence of the cosmic Phoenix Force entity, which had created a duplicate body of Jean, believed itself to be Jean and acted in her place while the real Jean lay in a healing cocoon at the bottom of Jamaica Bay, where the Avengers and Fantastic Four would later discover her.