As the humanistic psychology of Abraham Maslow became a "third force" after behaviorism and psychoanalysis, transpersonal psychology evolved as a "fourth force", extending humanistic psychology, at the least, with the spiritual dimension. Maslow and his colleague Anthony Sutich agreed with this formulation. Michael Harner added that it was "ethnocentric and cognicentric". 
It expands the psychological paradigm to include what has been called the "expanded mind", or a "suprarational, suprasensory level of mentation" that can be achieved with or without drugs. While shamanism commonly uses drugs to reach the shamanic state of consciousness, religions recognize general equivalents including satori in Zen Buddhism, the shema of Kabala, unio mystica in Catholicism, etc. Stanley Dean uses the term "ultraconsciouness" to encompass them.  A wide range of societies have traditions in which ritual produces trance, ecstatic, or other states of nonordinary reality. Transpersonal psychology works on creating contexts in which these fit.
Grof wrote that the Freudian model is inadequate to deal with
the dynamics of emotional and psychosomatic healing, personality transformation and consciousnesss evolution that come with certain powerful techniques, inclusinf psychedelic therapy, healing trance dance, or certain experiential techniques in modern psychotherapy. Such techniques activate and motivate deep unconscious and superconscious levels of the human psyche and require a greatly expanded conceptual framework. An individual that uses them for self-exploration or as a therapist has to have a model or cartography of the psyche that includes transbiographical domains.
His cartography includes one traditional psychotherapeutic domain, and two new ones:
- Perinatal domain, including near-birth and near-death experiences
- Transpersonal domain
The latter domains are, he says, accessible to most people, but with psychedelics or trance-inducing experiences, or in dreams.
Goodchild mentions the Sufi idea of "visionary states were thought to be 'really real'; these landscapes were called the mundus imaginalis, and were clearly distinguished from fantasy, meaning unreal, states." In a psychiatric context, Mack wrote of "Non-Ordinary States of Consciousness". 
Wilson Van Dusen, a clinical psychologist, categorized his patients' hallucinations, and came to believe that the work of Emmanuel Swedenborg (1688-1772), which categorized human experience as a series of interactions with spirits, was a model for Van Dusen's observations of hallucinations. 
Grof describes profound experiences with animals representing archetypes, which most commonly are events in the "inner world" but may be triggered by encounters with actual animals. In the Freudian model, these are symbolic, such as dreams of a predator representing aggression.
Animal identification, however, is considered uniquely transperonal. He quotes a typical statement of "No, you do not understand; there is nothing to analyze here, I really was an elephant. I know what an elephant feels like when he is angry or sexually aroused, and what it is like when water enters his trunk. An elephant does not stand for anything; an elepant is an elephant." This is different from an animal as spirit guide. 
In an article at the John Mack Institute, Virginia Goodchild wrote
It is possible, therefore, that the encounter experience is a contemporary form of an ancient mystical knowledge or gnosis, that is, knowledge that comes from the reality of visionary or revelatory states, that are also taking place in an actual "space" of the soul, or subtle vehicle. Such experiences also make it imperative that we expand our dichotomous worldview to include once again these other levels of reality, that in fact are by no means new, but recover an ancient multidimensionality.
- Stanislaus Grof, Brief History of Transpersonal Psychology, Al Jardim
- Sutich, A. (1976.) The emergence of the Transpersonal Orientation: A personal account. Journal of Transpersonal Psychology 8, 5-19.
- Michael Harner, The Way of the Shaman
- Stanley R. Dean (1974), 1: The Ultraconscious Mind, in John White, Frontiers of Consciousness: the meeting ground between inner and outer reality, Julian Press, p. 9
- Stanislav Grof (1988), The Adventure of Self-Discovery: (I: Dimensions of Consciousness; II: New Perspectives in Psychotherapy, State University of New York Press, ISBN 08877965414, pp. 1-2
- John E. Mack (1993), Chapter 16: Non-Ordinary States of Consciousness and the Accessing of Feelings, in Ablon, Steven; Brown, Daniel; Khantzian, Edward J., and Mack, John E., Human Feelings: Explorations in Affect Development and Meaning, The Analytic Press, at 357-371.
- Wilson van Dusen (1974), 4: Hallucinations and the world of the spirits, in John White, Frontiers of Consciousness: the meeting ground between inner and outer reality, Julian Press, p. 53
- Grof, pp. 118-121
- Veronica Goodchild, Alien Contact Experience and Ancient Traditions, John Mack Institute