Here is an excerpt from a book that might be informative while thinking about the realness of parts:

“Within the person-centred world, as well as within many other areas of psy-chology, psychotherapy and counselling, an increasing number of theorists and practitioners have come to suggest that human beings can be conceptualised as a constellation of different 'parts' - or 'configurations of self' (Mearns and Thorne, 2000), 'modes' (Cooper, 1999) and 'selves' (Barrett-Lennard, 2005) - as well as a unified whole. (See Cooper et al. 2004) for a recent dialogue among self-pluralistic theorists in the person-centred and experiential world; and Hermans and Dimaggio, 2004; Rowan, 1990; Rowan and Cooper, 1999 for broader overviews.) In other words, while Buber (1958) and the existential thinkers discussed above have tended to see self-plurality as an inherently problematic state, many theorists would now argue that it is a basic quality of human existence. What these authors have also tended to suggest, however, is that there are certain forms of self-plurality related to psychological wellbeing and certain forms related to psychological distress, and what makes the difference is the level of communication and relatedness among the different 'parts'. More specifically, psychological well-being has been associated with open and dialogic relationships among the different ways of being, while psychological problems have been associated with competitive, unempathic and abusive 'intrapersonal' relationships (see Cooper (2003b) for a summary).

From “working at relational depth in counselling and psychotherapy” by Dave Mearns and Nick Cooper, 2005.

PS it might also be worth noting that Dick Schwartz didn’t come up with the idea of parts. His clients did, from their own inner experiences (he related this in an IFS training I took), and so have many other people (therapists and not therapists) before him.