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Mind, Body & Soul: The Three Forces of Psychology

What does it mean to be human? This question has fascinated thinkers in every corner of the world and throughout recorded history. It has been the subject of countless works of philosophy, literature, art and science, and yet the question remains unanswered.

Humans are but animal, some have suggested (e.g. Darwin, Watson, Skinner), and we must be tempered with training and structure. We are conscious creatures, suggested others (e.g. Freud, James, Jung), and it is the depths of the mind that should be explored. Even so, a person is surely something more than just mind and body, observed others still (e.g. Maslow, May, Rogers). We are beings with souls that exceed the sum of our parts. This debate stretches into the ages, and continues now, because indeed all three perspectives are true. Each reflects an essential dimension of human existence – mind, body, and spirit.

Together these perspectives offer a holistic picture of what it means to be a human being endowed with a physical body, evolved with a thinking mind, and imbued with an ever-unfolding and transcendent sense of self. In the relatively recent age of science, each dimension has inspired one of three major movements in the field of psychology – Psychoanalysis, Behaviorism, and Humanistic Psychology.

While each force has been influential in its own right, none is able to fully illuminate the human experience on its own. We must understand all three if we are to grasp a more complete understanding of psychology and the nature human-kind.

The human psyche may have been a subject of interest throughout history, but the pursuit of a formal psychological science did not emerge until the latter half of the 19th Century. In Vienna, a medical doctor named Sigmund Freud began treating patients for mysterious symptoms that seemed to manifest from within the mind. Inspired by the practice of mesmerism, or hypnosis, Freud observed that the power of suggestion could be used to access hidden thoughts from a patient, or even alter how a person would later respond [10]. Freud expanded on this idea to develop his own method for examining the hidden realm of the mind. It was called Psychoanalysis.

Freud’s most significant and enduring contribution to psychology is his model of the mind consisting of the unconscious (which he called the id), the conscious (known as the ego), and the moral conscience (or super-ego) derived from the social environment [10, 11, 14]. According to Freud, the unconscious is home to man’s most primal desires and exists outside of awareness. This includes the sexual impulse, or libido, which was the focus of Freud’s theory and much of early psychoanalysis. As the ego and superego developed in childhood, Freud believed that early life experiences prompted the repression of certain libidinal desires that were nonetheless making demands on the adult psyche. This conflict was the source of many neurotic conditions.

As psychoanalysis continued to evolve, Freud would eventually expand his ideas about the unconscious. An aggressive impulse was added, although the pleasure principle would remain a central theme of his theory [14, 15]. This psycho-sexual focus became a key point of debate within the psychoanalytic school and the medical community more broadly [19,44]. A number of Freud’s contemporaries, including Carl Jung, Alfred Adler, Karen Horney, and Freud’s daughter, Anna, would eventually divert from his theories and explore alternative sources of psychodynamic conflict.

For Freud’s successors, the human psyche included needs of love, acceptance, and power, environmental aspects such as a person’s place in society or style of living [2, 16, 22, 23]. Their expanded focus also placed a much greater emphasis on the ego than Freud had, and some Neo-Freudians began to recognize additional stages of development in adulthood, which also elevated the struggle between the conscious mind and the external world of the superego.

Although the theories and practices of psychoanalysis were intriguing, the movement faced significant challenges from the scientific community. Most prominent was the demand for empirical evidence of an unconscious mind and the methods used to access it [29, 51]. Freud had consistently described his theories with the language of the natural sciences, which were dominant in his time, His use of terms like “drive” and “force” even evoked the positivist empiricism of physics [10]. In truth, the individualized nature of its methods and the limitations of technology at the time meant that psychoanalysis could not offer such evidence for its claims.

The desire for a more objective approach sparked the beginnings of another movement which emerged from the school of Wilhelm Wundt, founded in Leipzig, Germany, in 1879. Along with his protégé, Edward Titchener, at Cornell University in the United States, Wundt abandoned consideration of the unconscious in favor of examining only what could be observed through introspection [17, 40]. However, the Leipzig school’s focus on the contents of the mind would soon fall out of fashion.

As the study of psychology spread in the United States, it would become increasingly influenced by the evolutionary theories of Charles Darwin and the philosophy of pragmatism [17, 24]. These ideas inspired William James, a researcher at Johns Hopkins University, to publish his now classic volumes, Principles of Psychology, in which he built the foundations of American Psychology as a science concerned with the function of consciousness [28].

For the American Functionalists, biology and animal behavior became increasingly relevant as a source of knowledge, but this approach still required researchers to speculate about the intention of the animal or human under observation. Before long, these attempts to interpret consciousness were seen as preventing objective observation and limiting psychology from becoming a natural science [9, 40, 60]. Eventually, William James went so far as to ask if consciousness even existed, while others still held to the dualistic model of mind and body that had dominated psychology thus far.

Meanwhile, in Russia, Ivan Pavlov had discovered the curious ability to produce involuntary behavioral responses that required no guesswork to understand. He had trained dogs to salivate at the sound of a buzzer, demonstrating what became known as classical conditioning [26]. Pavlov’s work intrigued the functional psychologists when it was published in the United States, including John B. Watson, who would be inspired to write his famous manifesto, Psychology as the Behaviorist Views It [5, 60]. In this short but influential article, Watson proposed to transform psychology into a science concerned only with observable behavior.

Watson’s approach drew no distinction between animal and human, and framed psychology as a mechanistic, physiological science dealing with external phenomena that could be measured. He believed that understanding the relationship between stimulus and response (S-R) would allow science to predict how animals, including humans, would behave under similar conditions, and thus behavior could be controlled. However, a debate continued within the behavioral movement as some wondered if this kind of approach was adequate to explain human psychology.

Edward Tolman was among those who questioned behaviorism. As he observed, animals and humans were often unpredictable.58 To explain this variability, Tolman offered an alternative theory that inserted the organism into the S-R relationship (S-O-R). This approach examined the reason or intention for a behavior performed. Tolman had recognized an important limitation of Watson’s methodological behaviorism and its exclusion of learning, motivation and cognition, but his solution had also reintroduced mentalism into the equation. By interpreting the intentions behind a behavior, the Tolman and other Neo-Behaviorists were essentially introspecting again.

This issue was later addressed by B.F. Skinner, whose theory of radical behaviorismhelped to explain the full range of behavior in terms of S-R response [33, 40]. Skinner achieved this by introducing the concept of operant conditioning, which describes how voluntary, learned behaviors are reinforced by external as well as internal stimuli [55, 56]. This theory allowed behaviorism to explain even the more complex human behaviors, such as language and even consciousness itself [49, 55].

Through his research and writings, Skinner received considerable notoriety, and even a level of mainstream fame. He’d achieved the goal of developing an empirical and replicable science of psychology. However, in doing so, Skinner had reduced the field to little more than animal training. As such, psychology had lost its soul.

By the 1960s, the field of psychology was reexamined once again. As that decade’s sweeping cultural change prompted some to challenge established norms, the theories of psychoanalysis and behaviorism seemed inadequate to meet the pressing questions of the time [37]. Those earlier movements had indeed produced a great deal of knowledge but their focus on abnormality and mental illness left many qualities of the human condition unexplored [42, 54].

Some psychologists, like Abraham Maslow, hoped to extend the focus of psychology by instead directing research at individuals who were flourishing [35]. Maslow’s hierarchy of needs described a path from basic functioning toward peak performance, or self-actualization, and became a key theory in the third movement known as Humanistic, Transpersonal, and Existential (HTE) Psychology.

Humanistic psychology emerged from a pivotal meeting known as the Old Saybrook Conference, which took place in Connecticut in 1964. The assembled theorists advanced a new paradigm that recognized humans as having consciousness and able to consider their awareness of self and others subjectively [1, 46]. As such, human beings were described as uniquely capable of exceeding the mechanical inner workings of their bodies and minds.

Among the speakers, Carl Rogers challenged the underlying philosophies of Behaviorism, Rollo May presented on the concept of intentionality as a central aspect of human will, and Maslow explored transcendent experience and the actualizing potential of humans for which he is best known. In doing so, these leaders expanded the scope of psychology to embrace the subjective, individualistic nature of humanity, rather than to reduce and strip away this quality as had been done previously.

In order to achieve a scientific study of subjective experience, HTE rested its foundation on the ideas of German philosophers Edmund Husserl and Martin Heidegger, who developed ontological and epistemological theories based in the nature of being as an unfolding process of embodied existence [12, 42].

This meant that, unlike the previous psychological movements based in the positivist empiricism of the natural sciences, humanistic inquiry would utilize qualitative research methods that examine subjective experience and the meaning it produces for each individual at a specific place and time. From this perspective, HTE is able to explore how the context and systems within an individual’s environment influence his/her conceptualization of self [37].

Like the theories each put forward, the three major psychological movements also produced methods of treatment that were influenced by the norms and concerns of their times. As Freud was beginning his practice of psychoanalysis, the treatment of mental illness had not yet developed outside of asylums, which served more as wards to remove patients from society than centers of healing [25]. Under these circumstances, the cases seen by Freud were primarily those of aristocrats, often women, and persons of means and education. Thus the talking cure of psychoanalysis was born.

In psychoanalytic treatment, the therapist asks the patient to discuss memories and recall dreams in the process of free association [10, 11]. The psychoanalyst acts as a guide and interpreter looking for meaning and connection to the unconscious [43, 57]. The key element of this method is the dynamic between patient and therapist known as transference [20, 51].

According to Freud’s theory, the patient reacts to the therapist in the role of caregiver and authority figure, projecting unconscious associations onto the relationship which may reveal themes from childhood or other repressed thoughts. Likewise, the phenomenon of counter-transference occurs as the therapist may project his own unconscious onto the patient, which must be monitored carefully.

Psychodynamic talk therapy can take a great deal of time, and it is highly dependent on the patient’s ability to provide material for interpretation as well as process the insight gained. As such, this approach was not always best positioned to treat severe mental disorders or to deliver timely relief of symptoms.

By the time of the World Wars, behaviorism offered an attractive alternative for both the assessment of recruits and treatment of the more acute issues at hand in the aftermath of battle [17, 24]. Through behavior modification techniques, patients could be trained, quite literally, to reduce maladaptive behaviors and adopt more successful responses.

With the arrival of the computer age, behavioral treatments took on a new dimension in its updated model of the brain as a type of computer itself. Cognitive-Behavioral Therapy (CBT) maintains its focus on stimulus conditioning, but also recognizes the software-like programming of the mind which helps to determine responses [6, 27, 39]. For example, dysfunctional thoughts can run like routines in the brain, leading to symptoms of depression and anxiety [3]. Social experiences may also influence the individual in ways that inhibit motivation [8]. The CBT approach is to identify and challenge these responses, offering a systematic program for developing new and more productive ways of thinking and behaving.

Although CBT has been a successful and widely adopted method of treatment, there is a somewhat impersonal aspect to this type of behavioral intervention. In the light of humanistic thought, the prescriptive approaches of both psychoanalysis and behaviorism overlook the individual aspects of each patient in favor of an established norm. An alternative stance can be seen in the Person-Centered Therapy (PCT) of Carl Rogers, which influences much of humanistic treatment [47]. In this method, the patient is seen as the source of experience and meaning, and the therapist seeks only to guide the patient toward understanding and acceptance of the unique self.

Practices of mindfulness, yoga, and prayer are also utilized in humanistic therapy, with the goal of providing a holistic approach to healing that includes the mind, body, and soul of the individual.

The three major psychological movements — psychodynamic theory, behaviorism, and humanistic psychology — may first appear to be worlds apart. However, upon closer inspection, there is common ground to be found among these important forces.

Concern for the humanistic side of the equation can be seen in Freud’s methods, which recognized individual differences in perception, such a repression and free association, as a cause and treatment of neurotic symptoms. On the other hand, while behaviorism was largely opposed to any discussion of the mind, the insistence by some that consciousness and cognition should not be overlooked eventually led to the development and subsequent dominance of cognitive-behavioral psychology, where that domain’s closest connections with psychodynamic theory are found.

Psychoanalysis recognized that personality develops over time and life events may be internalized to influence relationships and experiences later in life. A similar concept is present in behaviorism, which recognizes the role of experience in conditioning behavioral responses. This is seen even more clearly in cognitive-behavioral psychology where social learning and environmental factors are considered as stimuli that contribute toward development of mental schemas and automatic thoughts which may occur outside the realm of consciousness. At a more basic level, these approaches also share with humanistic psychology a concept the ongoing and evolving process of being.

Humanistic, Transpersonal, and Existential psychology is understood as a highly relational practice, as is evidenced in the work of noted theorists, Carl Rogers and Rollo May. In this approach, the connection between the therapist and the client is seen as paramount to the success of therapy. Similarly, psychoanalysis is dependent on the relational process of talk therapy, including the central mechanism of transference. In modern psychoanalysis, it is now recognized that the patient should be encouraged to make associations and draw connections more independently and with fewer interpretations from the analyst [20, 43, 45]. This is supported by empirical evidence that suggests fewer interpretations made by the analyst results in more effective therapy [51].

Behaviorism was noted for its exclusive attention to stimuli and bodily responses. Even in its physiological rigidity, this perspective still shares some commonalities with its competing movements. For example, in psychoanalysis, Freud was attentive to the patient’s body during sessions when looking for nonverbal cues, but also in regard to the therapeutic posture that required a patient to lie on their back with the analyst seated behind and out of view. This position was not only to relax the patient, but also to prevent the therapist from influencing free associations with his own reactions. Likewise, a tenet of humanistic research and practice requires the investigator to bracket personal opinions out of the process.

The contributions of behaviorism are carried forward in HTE in other ways as well. Positive Psychology, which is often considered a subset of humanistic study, is rooted in the empirical and methodological foundations of behavioral science [52, 59]. Sharing with HTE the interest in expanding human performance, positive psychology focuses primarily on empirical outcomes and methods which can deliver more immediate changes in behavior to promote flourishing of the individual.

As psychology continues to evolve in this century, an integrative approach is key to the development of new theories, or perhaps the revival of ancient and more basic questions, which rest at the intersection of mind, body, and soul.

Since the first movement of psychoanalysis, it has been recognized that the mind is capable of producing injury to the body. Linking this notion with more recent perspectives, one might ask if the mind, or perhaps the spirit, can heal the body [13, 50].

Religious traditions have long maintained the connection between mind, body, and soul, and thus many practices that integrate these dimensions can be found in both Western and Eastern faith traditions. Yoga, as an example, is a physical practice which is shown to produce mental clarity and even spiritual experience [34]. The embrace of this type of therapy in humanistic psychology has influenced the broader psychological and medical communities, as well as the culture at large, all of which are increasingly exploring the healing connection between mind and body.

The transcendent spiritual benefits of religious faith and meditation are getting closer consideration as well. Questions about how these elements of human experience impact each other will require empirical study in the decades ahead, and each psychological movement is uniquely positioned to contribute to that knowledge.

As psychologists seek to address the needs and challenges of modern men and women, the answers to these questions, and the most profound discoveries yet to come, may be found at the intersections of these three psychological forces.

References for additional reading

  1. Aanstoos, C., Serlin, I. & Greening, T. (2000). History of Division 32 (Humanistic Psychology) of the American Psychological Association. In D. Dewsbury (Ed.), Unification through Division: Histories of the divisions of the American Psychological Association, Vol. V. Washington, DC: American Psychological Association.

  2. Adler, A. (1927). Practice and theory of individual psychology. New York: Harcourt.

  3. Beck, A. T. (2005). The Current State of Cognitive Therapy. Archives of General Psychiatry,62(9), 953-959. doi:10.1001/archpsyc.62.9.953

  4. Butler, A. C., Chapman, J. E., Forman, E. M., & Beck, A. T. (2006). The empirical status of cognitive-behavioral therapy: A review of meta-analyses. Clinical Psychology Review, 26(1), 17-31.

  5. Clark, R. E. (2004). The classical origins of Pavlov’s conditioning. Integrative Physiological and Behavioral Science, 39(4), 279-294.

  6. Cristea, I. A., Montgomery, G. H., Szamoskozi, S., & David, D. (2013). Key constructs in “classical” and “new wave” cognitive behavioral psychotherapies: Relationships among each other and with emotional distress. Journal of Clinical Psychology, 69(6), 584-599.

  7. Darwin, C. (1859). On the origin of the species by means of natural selection. London: John Murray.

  8. Deci, E. L., & Ryan, R. M. (2008). Facilitating optimal motivation and psychological well-being across life’s domains. Canadian Psychology, 49(1), 14-23.

  9. Dewey, J. (1896). The reflex ark concept in psychology. Psychological Review, 3, 357-370.

  10. Erdelyi, M. H. (1985). Psychoanalysis: Freuds cognitive psychology. New York: Freeman.

  11. Feist, J., Feist, G., & Roberts, T. (2013). Freud: Psychoanalysis. In J. Hadley, Psychoanalysis (pp. 54-103). New York, NY: McGraw-Hill.

  12. Felder, A. J., Aten, H. M., Neudeck, J. A., Shiomi-Chen, J., & Robbins, B. D. (2014). Mindfulness at the heart of existential-phenomenology and humanistic psychology: A century of contemplation and elaboration. Humanistic Psychologist, 42(1), 6–23.

  13. Fossarelli, P. (2002). Fearfully wonderfully made: The interconnectedness of body-mind-spirit. Journal of Religion and Health, 41(3), 207–229.

  14. Freud, S. (1961). Civilization and its discontents (J. Strachey, Trans.). New York: W. W. Norton. (Original work published 1930)

  15. Freud, S. (1964). The standard edition of the complete psychological works of Sigmund Freud (J. Strachey, Ed.). Oxford, England: Macmillan.

  16. Fromm, E. (1955). The sane society. New York: Rinehart.

  17. Green, C. D. (2009). Darwinian theory, functionalism, and the first American psychological revolution. American Psychologist, 64(2), 75-83. doi: 10.1037/a0013338.

  18. Giorgi, A. (2015). The self and humanistic psychology. In K. J. Schneider, J.F. Pierson & J.F.T. Bugental (Eds.), The handbook of humanistic psychology: Theory, research, and practice (pp. 61-71). Thousand Oaks, CA: SAGE.

  19. Hall, C. S., & Lindzey, G. (1957). Social psychological theories: Adler, Fromm, Horney, and Sullivan. In Theories of personality (pp. 114-156). Hoboken, NJ: John Wiley & Sons Inc. doi:10.1037/10910-004.

  20. Hebbrecht, M. (2013). The dream as a picture of the psychoanalytic process. Romanian Journal of Psychoanalysis, 6(2), 123–142.

  21. Hoffman, L., Stewart, S., Warren, D. & Meek, L. (2015). Toward a sustainable myth of self: An existential response to the postmodern condition. In K. J. Schneider, J.F. Pierson & J.F.T. Bugental (Eds.), The handbook of humanistic psychology: Theory, research, and practice (pp. 105-133). Thousand Oaks, CA: SAGE.

  22. Horney, K. (1942). Self-analysis. New York: Norton.

  23. Horney, K. (1945). Our inner conflicts. New York: Norton.

  24. Hothersall, D. (2004a). Functionalism at the University of Chicago and Columbia University. In J. Langdal, Behaviorism (pp. 2-33). New York, NY: McGraw-Hill.

  25. Hothersol, D. (2004b). The History of Clinical Psychology and the Development of Psychoanalysis. In J. Hadley, Psychoanalysis (pp. 3-53). New York, NY: McGraw-Hill.

  26. Hothersall, D. (2004c). The research of Ivan Pavlov and the behaviorism of John B. Watson. In J. Langdal, Behaviorism (pp. 2-33). New York, NY: McGraw-Hill.

  27. Hunt, R. R. & Ellis, H. C. (2004). Fundamentals of Cognitive Psychology. New York, NY: McGraw-Hill.

  28. James, W. (1890). Principles of Psychology (2 vols.). New York: Henry Holt.

  29. Johansson, M. (2007). Historiography and Psychoanalysis. International Forum of Psychoanalysis, 16(2), 103–112.

  30. Jung, C. G. (1966). The Relations Betweem the Ego and the Unconscious. Routlegde and K. Paul.

  31. Keddy, P. (2011). My Experience with psychotherapy, existential analysis, and Jungian analysis: Rollo May and beyond. Journal of Clinical Psychology, 67(8), 806–817.

  32. Klüners, M. (2014). Freud as a philosopher of history. Journal of Psychohistory, 42(1), 55–71.

  33. Ledoux, S. F. (2012). Behaviorism at 100. American Scientist, 100(1), 60-65.

  34. Luskin, F. (2004). Transformative practices for integrating mind-body-spirit. Journal of Alternative and Complementary Medicine, 10(Supplement), S15–s23.

  35. Maslow, A. (1968). Toward a psychology of being. New York: Van Nosgrand Reinhold.

  36. May, R. (1969). Love and will. New York: Norton.

  37. McDonald, M., & Wearing, S. (2013). A reconceptualization of the self in humanistic psychology: Heidegger, Foucault and the sociocultural turn. Journal of Phenomenological Psychology, 44(1), 37–59

  38. McLeod, J. (2013). An introduction to counselling. Maidenhead: Open University Press.

  39. Mischel, W., & Shoda, Y. (1995). A cognitive-affective system theory of personality: Reconceptualizing situations, dispositions, dynamics, and invariance in personality structure. Psychological Review, 102(2), 246-268.

  40. Moore, J. (2011). Behaviorism. The Psychological Record, 61(3), 449-464.

  41. Moore, J. (2013). Tutorial: Cognitive psychology as a radical behaviorist views it. The Psychological Record, 63(3), 667-680.

  42. Moss, D. (2015). The roots and genealogy of humanistic psychology. In K. J. Schneider, J.F. Pierson & J.F.T. Bugental (Eds.), The handbook of humanistic psychology: Theory, research, and practice (pp. 3-18). Thousand Oaks, CA: SAGE.

  43. Newirth, J. (2015). Psychoanalysis’ past, present, and future: Sherlock Holmes, Sir Lancelot, and the Wizard of Oz. Psychoanalytic Psychology, 32(2), 307-320. doi:10.1037/a0035251

  44. Overskeid, G. (2007). Looking for Skinner and finding Freud. American Psychologist, 62(6), 590–595.

  45. Perera, S. B. (2013). Circling, Dreaming, Aging. Psychological Perspectives, 56(2), 137–148.

  46. Polkinghorne, D. (2015). The self and humanistic psychology. In K. J. Schneider, J.F. Pierson & J.F.T. Bugental (Eds.), The handbook of humanistic psychology: Theory, research, and practice (pp. 87-104). Thousand Oaks, CA: SAGE.

  47. Rogers, C. (1961). On becoming a person: A therapists view of psychotherapy. Boston, MA: Houghton Mifflin.

  48. Rowan, J. (2015). Existential analysis and humanistic psychology. In K. J. Schneider, J.F. Pierson & J.F.T. Bugental (Eds.), The handbook of humanistic psychology: Theory, research, and practice (pp. 549-561). Thousand Oaks, CA: SAGE.

  49. Ruiz, M. R. (1995). B. F. Skinner’s radical behaviorism: Historical misconstructions and grounds for feminist reconstructions. Psychology of Women Quarterly, 19(2),161-179.

  50. Samuel, G. (2010). Healing, efficacy and the spirits. Journal of Ritual Studies, 24(2), 7–20.

  51. Schut, A. J., & Castonguay, L. G. (2001). Reviving Freud’s vision of a psychoanalytic science: Implications for clinical training and education. Psychotherapy, 38(1), 40–49.

  52. Schneider, K. (2011). Toward a humanistic positive psychology: Why can’t we just get along? Existential Analysis, 22(1), 32–38.

  53. Schneider, K. (2015). Rediscovering awe: A new front in humanistic psychology, psychotherapy, and society. In K. J. Schneider, J.F. Pierson & J.F.T. Bugental (Eds.), The handbook of humanistic psychology: Theory, research, and practice (pp. 73-81). Thousand Oaks, CA: SAGE.

  54. Seligman, M. E. P., & Csikszentmihalyi, M. (2000). Positive psychology: An introduction. American Psychologist, 55(1), 5–14.

  55. Skinner, B.F. (1957). Verbal behavior. New Jersey: Prentice Hall.

  56. Skinner, B.F. (1969). Contingencies of reinforcement: A theoretical analysis. New York: Appleton-Century-Crofts.

  57. Summers, F. (2006). Freud’s relevance for contemporary psychoanalytic technique. Psychoanalytic Psychology, 23(2), 327–338.

  58. Tolman, E. C. (1922). A new formula for behaviorism. Psychological Review, 29(1), 44-53.

  59. Waterman, A. (2013). The humanistic psychology – positive psychology divide: Contrasts in philosophical foundations. American Psychologist, 68(3), 124–133.

  60. Watson, J. B. (1913). Psychology as the behaviorist views it. Psychological Review, 20(2), 158-177. doi:10.1037/h0074428

  61. Weaver, Y. (2009). Mid-life – A time of crisis or new possibilities? Existential Analysis, 20(1), 69–78.


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