Multiple theories of adult development have synthesized several systems at the higher levels of cognition. To think in multiple systems, you must actually understand the different systems.
In a previous article, I briefly summarized Piaget’s theory of child development(1936) which is notable for focusing on a hierarchical cognitive development where more advanced stages incorporate and extend previous stages. Piaget’s theory ended around the preteens when children developed abstract concepts, theories, and systems. Of course, there are other models of how it could be, but the two theories discussed here are considered neo-Piagetan because they have the same structure extended into adulthood.
Keagan's Theory of Development
Keagan’s theory of adult development is proposed in his 1986 book, The Evolving Self: Problem and Process in Human Development. It outlines a 5 stage progression of consciousness that influences how people understand themselves and the world. A core idea is the Subject-Object Relationship at each stage where there are aspects of our experience that we are "subject to" (embedded in and unable to see objectively) and aspects that we can take as "object" (can reflect upon and control).
Here's a list of Kegan's five orders of consciousness with brief explanations:
Impulsive (1st Order, Early Childhood):
Subject(unchanging): Impulses, perceptions
Object(controllable): Reflexes, sensations
Ego-centric
Imperial (2nd Order, Most Children, Some Adults):
Subject: Needs, interests, wishes
Object: Impulses, perceptions
Develops a sense of self with needs, wishes, and interests, but limited empathy and transactional view of relationships
Socialized/Interpersonal (3rd Order, Most adults):
Subject: Interpersonal relationships, mutuality
Object: Needs, interests, wishes
NPCS, identity defined by relationships and social roles
Self-Authoring/Institutional (Some adults):
Subject: Self-authorship, identity independent of relationships, ideology
Object: Interpersonal relationships, mutuality
Consistent, independent identity/value system and ability to take responsibility for inner states and emotions
Self-Transforming/Inter-individual (Very few adults):
Subject: Interindividuality, interpenetration of systems
Object: Self-authorship, identity, ideology
Not tied to a particular identity so often integrates multiple systems of meaning despite the paradox and contradiction
Capacity for true intimacy while maintaining autonomy
And next up we have the MHC theory, but for that, we need another article.