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This article was written by an Expert contributor and is protected from general editing. There is a Community article on this subject at: Tree of Knowledge System.

The Tree of Knowledge (ToK) System is a novel, theoretical approach to the unification of psychology developed by Gregg Henriques, Ph.D., Assistant Professor, Combined-Integrated Doctoral Program, James Madison University, Harrisonburg, VA, USA.

The outline of the system was published in 2003 in Review of General Psychology and two special issues of the Journal of Clinical Psychology in Dec 04 and Jan 05 were devoted to the elaboration and evaluation of the model.

The problem of psychology

The most difficult problem in psychology as a discipline is that while there is incredible diversity offered by different approaches to psychology, there is no overall consensus model of what Psychology actually is. According to the ToK System, the problem of psychology is that a clear definition, an agreed upon subject matter, and a coherent conceptual framework have eluded its students for its entire 125 year history. The patent tendency (of psychology) has been toward theoretical and substantial fractionation and increasing insularity among the “specialties.” In other words, the discipline has fragmented into different schools of thought and methodology, with no overall framework to interpret and integrate the research of different areas. At its best, the different approaches are a strength of psychology; different approaches lead to novel ideas, and prevent psychologists from clinging to a paradigm that fails to explain a phenomena. At its worst, adherents of one particular school cling to their beliefs concerning the relative importance of their research and disregard or are ignorant of different approaches. In most cases, the individual psychologist has to determine for themselves which elements of which perspective to apply, and how to integrate them into their overall understanding.

The reason for psychology’s fractionation, according to the ToK, is that there has been no meta-theoretical frame that allows scholars to agree on the basic questions that need to be addressed. As such, the different schools of thought in psychology are like the blind men who each grab a part of the elephant and proclaim they have discovered its true nature. With its novel depiction of evolving dimensions of complexity, the ToK allows scholars to finally see the elephant. In the 2003 Review of General Psychology paper, Henriques used the ToK System to clarify and align the views of B.F. Skinner and Sigmund Freud. These luminaries were chosen because when one considers their influence and historical opposition, it can readily be argued that they represent two schools of thought that are the most difficult to integrate. Henriques used the meta-perspective offered by the ToK to show how one can retain the key insights from each school of thought, identify errors and points of confusion, and integrate the insights into a coherent whole.

Tree of Knowledge


The Tree of Knowledge Diagram. Adapted from Presentation on The Official ToK System website.

In one way, the Tree of Knowledge (ToK) System reflects a fairly common hierarchy of nature and of the sciences that has been represented in one way or another since the time of Augusta Comte, who in the seventeenth century used a hierarchical conception of nature to argue for the existence of sociology. Despite its surface agreement with a standard conception, in actuality the ToK System offers a set of novel ideas that have crucial implications for both ontology and epistemology. The novel ontological claim made by the ToK, and depicted pictorially above, is that cosmic evolution consists of four separable dimensions of complexity, namely Matter, Life, Mind, and Culture. The dimension of complexity argument is one of the most intriguing, original and complicated aspects of the system. Many have argued nature is hierarchically leveled; for example, a list of such levels might be subatomic particles, atoms, molecules, cells, organ structures, multi-celled organisms, consciousness, and society is common. The ToK System embraces a view of nature as levels, but adds the notion that there are also dimensions of complexity. The difference is most clearly seen pictorially. A view of nature as solely consisting of levels would have a single cone of complexity, whereas the ToK depicts four cones. The ToK posits that a separate dimension of complexity emerges when a process of selection operates on a unit of information. Thus, according to the ToK, natural selection operating on genetic combinations gives rise to the dimension of Life; behavioral selection operating on neuronal combinations gives rise to the dimension of Mind; and justification operating on symbolic combinations gives rise to Culture.

The ToK System also offers a new epistemology that Henriques believes will move toward what E.O. Wilson termed consilience. Consilience is the interlocking of fact and theory into a coherent, holistic view of knowledge. The ToK affords new perspectives on how knowledge is obtained because it depicts how science emerges out of culture and that the four dimensions of complexity correspond to four broad classes of science: the physical, biological, psychological and social sciences.


The dimension of matter refers to the set of material objects and their behaviors through time. In accordance with modern cosmology, matter is theorized to have emerged out of a pure energy singularity at Big Bang. Space and time were also born at such a point. Matter is frozen chunks of energy. Nonliving material objects range in complexity from subatomic particles to large organic molecules. The physical sciences (i.e., physics, chemistry, geology, astronomy) describe the behavior of material objects.


The dimension of life refers to organisms and their behaviors through time. Living objects are considered a unique subset of material objects. Just as quantum particles form the fundamental units of material complexity, genes are the fundamental units of living information. Although many questions about the emergence of life remain unanswered, in accordance with modern biology, the ToK posits that natural selection operating on genetic combinations through time is the unified theory of biology and forms the foundational understanding for the emergence of organic complexity.


Mind in the ToK System refers to the set of mental behaviors. Mental behaviors are behaviors of animals mediated by the nervous system that produce a functional effect on the animal-environment relationship. As such, Mind is essentially synonymous with what behavioral psychologists have meant when they use the term behavior. Thus, a fly avoiding a fly swatter, a rat pushing a bar or a human getting a drink of water are all mental behaviors. Mind is not synonymous with sentience or the capacity for mental experience, although such processes are presumed to emerge in the mental dimension. Cognition, in the broad sense of the term (i.e., as meaning neuro-information processing) is seen as covert mental behavior, whereas change between the animal and the environment is defined as overt mental behavior. Defining mind as mental behavior provides a way to bridge the epistemological differences between cognitive and behavioral science.


Culture in the ToK System refers to the set of sociolinguistic behaviors, which range from large scale nation states to individual human justifications for particular actions. Just as genetic information processing is associated with the Life dimension and neuronal information processing associated with the Mind dimension, symbolic information processing emerges with the Cultural dimension.

Theoretical joint points

Quantum Gravity

Main article: Quantum Gravity at Wikipedia

Quantum Gravity refers to the imagined merger between the twin pillars of physical science which are quantum mechanics, the study of the microscopic (e.g., electrons), and general relativity, the science of the macroscopic (e.g., galaxies). Currently, these two great domains of science cannot be effectively interwoven into a single, physical Theory of Everything. Yet progress is being made, most notably through string theory, loop quantum gravity, black hole thermodynamics and the study of the early universe. Some of the difficulties combining these two pillars of physical science are philosophical in nature and it is possible that the macro view of knowledge offered by the ToK may eventually aid in the construction of a coherent theory of quantum gravity. The reason the ToK might help is that it locates scientific knowledge in relationship to the physical universe.

The Modern Synthesis

Main article: The Modern Synthesis

The Modern Synthesis refers to the merger of genetics with natural selection which occurred in the 1930s and 1940s and offers a reasonably complete framework for understanding the emergence of biological complexity. Although there remain significant gaps in biological knowledge surrounding questions such as the origin of life and the emergence of sexual reproduction, the modern synthesis represents the most complete and well-substantiated joint point.

Behavioral Investment Theory

Main article: Behavioral Investment Theory

Behavioral Investment Theory (BIT) is proposed as a merger of the selection science of behaviorism with the information science of cognitive neuroscience (notice the parallel with the modern synthesis). BIT posits that the nervous system evolved as an increasingly flexible computational control system that coordinates the behavioral expenditure of energy of the animal as a whole. Expenditure of behavioral energy is theorized to be computed on an investment value system built evolutionarily through natural selection operating on genetic combinations and ontogenetically through behavioral selection operating on neural combinations. As such, the current behavioral investments of the animal are conceptualized as the joint product of the two vectors of phylogeny and ontogeny. A unique element of BIT is that it finds a core of agreement and builds bridges between five brain-behavior paradigms: (1) cognitive science; (2) behavioral science; (3) evolutionary theory and genetics; (4) neuroscience; and (5) cybernetics/systems theory.

Justification Hypothesis

Main article: The Justification Hypothesis

The Justification Hypothesis (JH) is a novel proposal that allows for both the understanding of the evolution of culture and for identifying what makes humans distinct animals. A basic initial claim of the JH is that the process of justification is a crucial component of human mental behavior at both the individual and societal level. Unlike all other animals, humans everywhere ask for and give explanations for their actions. Arguments, debates, moral dictates, rationalizations, and excuses all involve the process of explaining why one’s claims, thoughts or actions are warranted. In virtually every form of social exchange, from warfare to politics to family struggles to science, humans are constantly justifying their behavioral investments to themselves and others.

The JH can be stated succinctly as follows: The evolution of language gave rise to the problem of justification, and this evolutionary pressure ultimately resulted in the human self-consciousness system and human culture. The JH carries with it three fundamental postulates.

  • The first is that the evolution of language must have created the problem of justification, which is the problem of explaining one’s self to others in a justifiable manner.
  • The second postulate is that the human self-consciousness system can be understood as a “justification filter”. This second claim links the evolutionary analysis with key insights from psychodynamic theory. Specifically, psychodynamic theory posits that socially unjustifiable impulses are inhibited and socially justifiable reasons are given for actions taken.
  • The third postulate is that culture can be understood as large scale justification systems that coordinate the behavior of human populations. Cultural systems are seen to evolve much in the same way as organisms do in biological evolution: there is a process of variation, selection and retention of belief systems.

How the ToK solves the problem of Psychology

The problem of psychology is its conceptual incoherence, which is clearly identifiable by the following:

(1) There is no agreed upon definition.
(2) There is no agreed upon subject matter.
(3) There is a proliferation of overlapping and redundant concepts.
(4) There are a large number of paradigms with fundamentally different epistemological assumptions.
(5) Specialization continues to be increasingly emphasized at the expense of generalization and thus the problem of fragmentation only grows.

When the various conceptions of psychology (e.g., behavioral, humanistic, cognitive) are viewed through the lens of the ToK System, it becomes readily apparent that psychology spans two different dimensions of complexity: the mental and the cultural. In other words, the discipline has historically spanned two fundamentally separate problems:

(1) the problem of animal behavior in general, and
(2) the problem of human behavior at the individual level.

If, as previously thought, nature simply consisted of levels of complexity, psychology would not be crisply defined in relationship to biology or the social sciences. And, indeed, it is frequently suggested that psychology exists in an amorphous space between biology and the social sciences. However, with its dimension of complexity depiction, the ToK System suggests that psychology can be crisply defined as the science of mind, which is the third dimension of complexity. Furthermore, because human behavior exists in the fourth dimension, psychology must be divided into two broad scientific domains of

(1) psychological formalism and
(2) human psychology.

Psychological formalism is defined as the science of mind and corresponds to the behavior of animal objects. Human psychology is considered to be a unique subset of psychological formalism that deals with human behavior at the level of the individual. Because human behavior is immersed in the larger socio-cultural context (level four in the ToK System), human psychology is considered a hybrid discipline that merges the pure science of psychology with the social sciences. It is important to point out that there are other disciplines the ToK System would classify as “hybrids.” Molecular genetics, for example, is a hybrid between chemistry and biology and neuroscience is a hybrid between biology and psychology. As with Henriques' proposed conception of human psychology, both of these disciplines adopt an object level perspective (molecular and cellular, respectively) on phenomena that simultaneously exist as part of meta-level system processes (life and mind, respectively).

Consciousness and Human Behavior

A frequent question and point of confusion in the ToK System is the definition and meaning of consciousness. As mentioned above, mind is not synonymous with consciousness. And, to understand consciousness from a ToK vantage point, it is crucial to recognize that the term is often ambiguous in its meaning. Two primary meanings are sentience, which is the capacity for mental experience and self-awareness, which is the capacity to be aware of one’s awareness. Sentience is conceptualized as a “level 3” phenomena, possessed by many animals other than humans and is defined as a “perceived” electro-neuro-chemical representation of animal-environment relations. The ingredient of neurological behavior that allows for the emergence of mental experience is considered the “hard” problem of conscious and the ToK System does not address this question explicitly. In contrast, through the Justification Hypothesis (see below), the ToK System involves a very direct analysis of the other issue of consciousness, that of self-awareness.

Another frequent question that is raised is “Where does individual human behavior fall on the ToK?” To analyze human behavior from the context of the ToK, one uses the ToK like a prism to separate the dimensions of behavior into physiochemical, biogenetic, neuropsychological and sociolinguistic. Thus if we imagine a conversation between a husband and wife as follows:

Wife: “You are late again.”
Husband: “Please not now. It was a stressful day and traffic was bad and you know if work needs to be done, I can’t just leave it.”

The words represent the sociolinguistic dimension and are understood as a function of justification. Justification systems are seen both at the level of individual, micro-social and societal (i.e., the context of justification in which men work and women stay at home). The actions of the husband and wife in terms of facial expression, body movement, etc. are seen as the mental dimension and are understood as a function of behavioral investment. The physiological make up of the organ systems and cells of each body is seen as the biogenetic dimension. Finally, the position, temperature, molecular make up is seen as the physiochemical dimension. Each of the more basic dimensions represent conditions of possibility that allow for the emergence of the higher dimension of process. Thus, insufficient oxygen disrupts organic processes which in turn renders neuropsychological and sociolinguistic processes impossible.

Toward the Integration of Human Knowledge

The ToK System also offers a new epistemology that should move academic knowledge toward what E.O. Wilson termed consilience. Consilience is the interlocking of fact and theory into a coherent, holistic view of knowledge. The ToK affords new perspectives on how knowledge is obtained because it depicts how science emerges out of culture and that the four dimensions of complexity correspond to four broad classes of science: the physical, biological, psychological and social sciences.

Developing such a system for integrating knowledge is not just an academic enterprise. Indeed, in an increasingly complex world, the fragmented state of knowledge can be seen as one of the most pressing social problems of our time. As history seems to attest, the absence of a collective worldview ostensibly condemns humanity to an endless series of conflicts that inevitably stem from incompatible, partially correct, locally situated justification systems. Thus there are good reasons for believing that if there was a shared, general background of explanation humanity, we might be able to achieve much greater levels of harmonious relations.

In 1958, Oliver Reiser issued a call for unifying scientific knowledge that remains as relevant today as it was a half a century ago:

In this time of divisive tendencies within and between the nations, races, religions, sciences and humanities, synthesis must become the great magnet which orients us all…[Yet] scientists have not done what is possible toward integrating bodies of knowledge created by science into a unified interpretation of man, his place in nature, and his potentialities for creating the good society. Instead, they are entombing us in dark and meaningless catacombs of learning.

With its novel depiction of the dimensions of complexity and interlocking theoretical joint points, the ToK System offers new avenues that might allow us meet Reiser’s poignant call. For with a shared sense of purpose and a common background of explanation, we might yet be able to integrate bodies of knowledge into a unified interpretation of humanity, our place in nature and our potentialities for creating the good society.

See also

References & Bibliography

Key texts



Additional material



  • Geary, D. C. (2005). The motivation to control and the origin of mind: Exploring the life-mind joint point in the tree of knowledge. Journal of Clinical Psychology, 61, 21-46. Full text
  • Gilbert, P. (2004). A much needed macro level view: A commentary on Henriques’ psychology defined. Journal of Clinical Psychology, 60, 1223-1226. Abstract
  • Haaga, D.A.F. (2004). Defining psychology: What can it do for us? Journal of Clinical Psychology, 60, 1227-1230. Abstract
  • Hayes, S.C. (2004). Taxonomy as a contextualist views it. Journal of Clinical Psychology, 60, 1231-1236. Abstract
  • Henriques, G.R. (2005). A new vision for the field: Introduction to the second special issue on the unified theory. Journal of Clinical Psychology, 61, 3-6. Full text
  • Henriques, G.R. (2005). Toward a useful mass movement. Journal of Clinical Psychology, 61, 121-139. Full text
  • Henriques, G.R., & Cobb, H.C. (2004). Introduction to the special issues on the unified theory. Journal of Clinical Psychology, 60, 1203-1205. Full text
  • Henriques, G.R., & Sternberg, R. J. (2004). Unified professional psychology: Implications for combined-integrated doctoral training programs. Journal of Clinical Psychology, 60, 1051-1063. Full text
  • Jones, R. (2005). From that dirty little science grows a Tree of Knowledge. The Madison, 1, 36-45. Full text
  • Katzko, M.W. (2004). Psychology’s dilemma: An institutional neurosis? Journal of Clinical Psychology, 1237-1242. Abstract
  • Kihlstrom, J.F. (2004). Unity within psychology, and unity between science and practice. Journal of Clinical Psychology, 60, 1243-1247. Abstract
  • Lilienfeld, S.O. (2004). Defining psychology: Is it worth the trouble? Journal of Clinical Psychology, 60, 1249-1253. Abstract
  • Mayer, J.D. (2004). How does psychotherapy influence personality? A theoretical integration. Journal of Clinical Psychology, 60, 1291-1315. Full text
  • Presbury, J. (2004). Rooting the tree of knowledge: A response to Henriques’ psychology defined. Journal of Clinical Psychology, 60, 1255-1258. Abstract
  • Quackenbush, S.W. (2005). Remythologizing culture: Narrativity, justification, and the politics of personalization. Journal of Clinical Psychology, 61, 67 - 80. Abstract
  • Rand, K.L., & Ilardi, S.S. (2005). Toward a consilient science of psychology. Journal of Clinical Psychology, 61, 7-20. Abstract
  • Shaffer, L.S. (2005). From mirror self-recognition to the looking glass self: Exploring the justification hypothesis. Journal of Clinical Psychology, 61, 47-65 . Abstract
  • Shealy, C.N. (2005). Justifying the justification hypothesis: Scientific-humanism, Equilintegration (EI) Theory, and the Beliefs, Events, and Values Inventory (BEVI). Journal of Clinical Psychology, 61, 81-106. Full text
  • Slife, B. (2005). Testing the limits of Henriques’ proposal: Wittgensteinian lessons and hermenuetic dialogue. Journal of Clinical Psychology, 61, 107-120. Abstract
  • Stam, H.J. (2004). Unifying psychology: Epistemological act or disciplinary maneuver? Journal of Clinical Psychology, 60, 1259-1262. Full text
  • Stanovich, K.E. (2004). Metarepresentation and the great cognitive divide: A commentary on Henriques' "Psychology Defined". Journal of Clinical Psychology, 60, 1263-1266. Full text
  • Stricker, G. (2004). The unification of psychology and psychological organizations. Journal of Clinical Psychology, 60, 1267-1269. Abstract
  • Vazire, S., & Robins, R.W. (2004). Beyond the Justification Hypothesis: A Broader Theory of the Evolution of Self-Consciousness. Journal of Clinical Psychology, 60, 1271-1273. Full text
  • Viney, W. (2004). Pluralism in the sciences is not easily dismissed. Journal of Clinical Psychology, 60, 1275-1278. Abstract
  • Yanchar, S.C. (2004). Some discontents with theoretical unification. Journal of Clinical Psychology, 60, 1279-1281. Abstract

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