The current study of the mind expands past philosophy to focus heavily on the sciences, although philosophical investigation about the mind and even about the sciences that study it are well and growing stronger each day. The interaction between these modes of inquiry has given rise to particular disciplines that no longer approach the full range of questions about the mind, but instead focus on specific frameworks, topics, and debates informed by philosophical reflection and current scientific knowledge. It is difficult, however, to determine precisely where one discipline ends and another begins, or to reconstruct a historical lineage by which one specifically gave rise to another. This ambiguity is reflected in the diversity of contemporary academic programs, each of which organizes mind studies according to their theoretical commitments and educational priorities.

In hopes of providing some clarity to those venturing into the study of the mind, I have put together a visual taxonomy of how I understand its disciplines. It is meant for anyone who has ever wondered, for example, what distinguishes neurophilosophy from philosophy of neuroscience, why psychology is not the same as philosophy of psychology, or how the philosophy of cognitive science differs from philosophy of cognition. The general taxonomy proposed here makes no claim to reconstruct the historical development of mind studies. It offers, however, an organization of the disciplines as they exist today, grouped according to their primary modes and objects of inquiry and the relationships that emerge among them within the contemporary landscape of mind studies. This is, of course, a work limited by the knowledge of a single individual whose audacity to map the disciplinary landscape came from reading an unholy amount of Oxford Handbooks. It should be read as a first draft. Respectful criticism, observations, and recommendations are welcome.

PSA: To define is to reduce. In order to map the disciplinary landscape of mind studies, I had to characterize modes of inquiry, objects of inquiry, and disciplines in ways that inevitably leave out a vast number of conceptions and, in many cases, conflict with them in principle. We can get all meta about it, but that’s just how I conceive them.

Diagram legend

Solid blue arrows indicate a foundational relationship between the elements of the taxonomy. They connect a mode of inquiry to the object toward which it is directed, an object of inquiry to the disciplines concerned with its study, or one discipline to another when the latter follows from or presupposes the former.

Dotted red lines indicate ongoing intellectual exchange. They connect disciplines that are in communication by drawing from one another’s questions, debates, methods, theories, concepts, or findings. These relationships reflect mutual influence and interdisciplinary dialogue. The common boundary enclosing sections IIIb, IVa, and IVb represents the same relationship denoted by the dotted red lines. It indicates that these disciplines are in continuous dialogue. I omitted individual connections for the sake of readability.

I. Modes of inquiry

Modes of inquiry are systematic approaches to investigating reality, distinguished by their aims, methods, and standards of evidence. Within the present taxonomy, I identified two modes of inquiry.

Philosophy is concerned with the formal, conceptual, normative, and ontological dimensions of investigation. It analyses what a phenomenon is, how it ought to be understood, and which concepts, assumptions, or frameworks are required to make sense of it.

Science is concerned with the empirical determination of how the world behaves within philosophical frameworks. Its goals are to describe, explain, and predict phenomena through systematic observation and experimentation.

II. Objects of inquiry

Objects of inquiry are the subject matters towards which a mode of inquiry is directed. They constitute the phenomena, systems, and entities investigated by particular disciplines. Within this taxonomy, I identified six objects of inquiry: science, mind, behavior, nervous system, experience, and cognition.

Science refers to the systematic enterprise of producing and evaluating empirical knowledge about the world. It appears as an object of inquiry because it is itself examined by Philosophy of Science and their specialized branches.

Mind refers to the totality of mental phenomena. It serves as the central object of inquiry for the majority of disciplines represented in this taxonomy.

Behavior refers to the actions and responses of an organism. It constitutes an object of inquiry in its own right, independently of any particular account of the mind.

The nervous system refers to the biological system responsible for processing and transmitting information within an organism. It grounds the disciplines concerned with the biological basis of mind and behavior.

Experience and cognition are included because of the role they play within the taxonomy: experience as the central object of inquiry of phenomenology, and cognition as the central object of inquiry of cognitive science. Their placement reflects what I consider to be the least contentious position, not a definitive claim about their ontological status.

IIIa. Foundational philosophical disciplines

Foundational philosophical disciplines are those that happen from applying philosophy as a mode of inquiry to the primary objects of inquiry identified in this taxonomy. Philosophy directed toward science constitutes the philosophy of science, while philosophy directed toward the mind constitutes the philosophy of mind. I consider them foundational because they provide the conceptual basis from which the remaining philosophical disciplines represented in this taxonomy are organized.

Philosophy of science investigates the nature of science and scientific knowledge. It asks what distinguishes scientific inquiry from other forms of inquiry, how scientific knowledge is generated and justified, and how scientific theories explain and represent the world.

Philosophy of mind investigates the nature of the mind and its place in the natural world. It asks what the mind is, how it relates to the natural world, what its fundamental phenomena are, and how they ought to be understood.

IIIb. Scientific disciplines

Scientific disciplines are those that result from applying science, as a mode of inquiry, to the primary objects of inquiry identified in this taxonomy. Science directed towards the mind, behavior, nervous system, and cognition constitutes the scientific disciplines represented here. Their primary aim is to produce empirical knowledge about these objects through systematic observation, experimentation, and formation of theories.

Psychiatry investigates the clinical characterization, explanation, and treatment of mental disorders. Its goal is to continuously characterize psychopathology, explain their mechanisms, and develop effective diagnostic and therapeutic approaches.

Psychology investigates behavior and mental processes across individuals and contexts. It aims at describing and characterizing behavior and mental processes, explaining their mechanisms and determinants, and predicting behavior and mental functioning.

Neuroscience investigates the organization, function, and dysfunction of the nervous system. Its goal is to characterize the nervous system across levels of biological organization, explain the organization and integration of neural systems, and advance the understanding of nervous system function and dysfunction.

Cognitive science investigates cognition through an interdisciplinary approach. It aims to characterize cognition across levels of analysis, explain the organization and function of cognitive systems, and advance the understanding of cognition through the integration of multiple disciplines.

Medical sience and computer science are included because of the foundational role they play within the taxonomy. They are not represented in detail because they do not constitute part of the primary focus of mind studies. They provide to be, however, essential foundations for disciplines that do. Medical science underlies psychiatry and computer science does cognitive science.

IIIc. Method-based philosophical disciplines

Method-based philosophical disciplines emerge from particular modes of inquiry directed toward particular objects of inquiry, but their primary aim is to complement or transform philosophy as a mode of inquiry. Unlike the remaining philosophical disciplines in this taxonomy, they are organized around distinctive methods of doing philosophy rather than around a particular philosophical object of inquiry. For this reason, they appear at the top of the taxonomy where they can influence philosophical inquiry across multiple domains.

Neurophilosophy complements philosophical inquiry through the findings, methods, and theoretical frameworks of neuroscience.

Experimental philosophy complements philosophical inquiry through empirical methods, drawing primarily from cognitive science and, more broadly, from the scientific enterprise.

Phenomenology complements philosophical inquiry through the systematic analysis of lived experience, using experience itself as its primary methodological foundation.

IVa. Mind-specific philosophical disciplines

Mind-specific philosophical disciplines are concerned with the philosophical investigation of particular objects within the philosophy of mind. They emerge from philosophical inquiry directed toward specific aspects of the mind.

Philosophy of consciousness investigates the nature of conscious experience.

Philosophy of cognition investigates the nature, structure, and assumptions underlying cognition.

Philosophy of perception investigates the nature and epistemic role of perceptual experience.

Philosophy of agency investigates the nature and mechanisms of agency.

Philosophy of emotion investigates the nature, explanation, and significance of emotions.

Philosophy of intentionality investigates the nature and sources of mental content.

Philosophy of self and personal identity investigates the nature of the self and individual identity.

IVb. Philosophies of scientific disciplines

Philosophies of scientific disciplines are concerned with the philosophical dimensions of particular scientific disciplines. They do not investigate empirical objects directly, as that is the role of the scientific disciplines from which they emerge, but analyse the frameworks, assumptions, methods, explanations, and epistemic foundations through which the disciplines operate.

Philosophy of psychiatry investigates the philosophical frameworks through which psychiatry operates.

Philosophy of psychology investigates the philosophical frameworks through which psychology operates.

Philosophy of neuroscience investigates the philosophical frameworks through which neuroscience operates.

Philosophy of cognitive science investigates the philosophical frameworks through which cognitive science operates.

Why a taxonomy?

A few months ago I finished working on a philosophy paper on the scientific study of emotions, particularly those generated by body mutilation images. I argued that the common characterization of these emotions as either disgust or fear was inadequate to account for the neural, psychological, and behavioral differences observed when individuals witnessed images involving the destruction of a body. I proposed the term horror to account for those differences. The point is that my co-author asked me what part of philosophy we were focusing on. Were we doing philosophy of neuroscience? Philosophy of emotion? Philosophy of cognitive science? Philosophy of science? My response was that we were taking from all of them to varying degrees, because the problem required us to engage with the concepts and debates concerning each of them.

That conversation was the main motivation for putting this taxonomy together. I think its value lies in making it easier to identify where one is standing when asking a complex question about the mind and to see which disciplines might have something useful to contribute. For example: questions about psychiatric nosology are informed by debates on natural kinds and construct validity; discussions about moral responsibility could benefit from research on executive control in the brain; theories of perception are informed by both neuroscientific evidence and phenomenological analyses of experience. Very few questions about the mind belong entirely to a single discipline, and having a map, I think, makes it easier to know where to look next.

The myth

The slogan has been circulating for several years now. I first encountered it while I was still an undergraduate studying neuroscience, sometime around 2021. Since then, it has become one of the internet’s favorite pieces of neuroscience. Hardly surprising in my opinion, since findings about the brain have a way of escaping the laboratory and entering pop culture as explanations for psychological phenomena they were never meant to explain. Researchers typically limit their conclusions to what their evidence can actually support, but pop science communicators rarely operate with the same restraint. On today’s internet, anyone can become a “neurojournalist” [1], regardless of whether they understand —or even care about— the science they are reporting. This gives rise to the familiar narrative that the prefrontal cortex fully develops by age twenty five, and with it arrives better judgment, emotional regulation, self control, responsibility, and other traits associated with psychological adulthood.

In order to determine whether this popular belief is true, we first need to decompose it into the distinct claims that are implicitly bundled into the phrase “the prefrontal cortex fully develops by 25.” The following propositions are not taken from a single source but reconstructed from years of observing how the phrase is used across social media:

1. There is such a thing as a fully developed brain, and this state is taken to represent brain maturity.

2. The prefrontal cortex reaches that state by age twenty five.

3. Because the prefrontal cortex is taken to underlie judgment, emotional regulation, personality, and social behavior, greater biological development of the prefrontal cortex corresponds to greater performance in these domains, and conversely.

4. Reaching the developmental endpoint of the prefrontal cortex marks the transition into adult psychological maturity.

Although the wording varies across the content I substracted these from, the underlying assumptions remain, in my opinion, fundamentally equal. I will evaluate each of them separately.

Claim #1: There is such a thing as a fully developed brain, and this state is taken to represent brain maturity.

It is worth distinguishing between the concepts of development and maturity. Development is a process, whereas maturity is a state. To say that something develops is to describe change over time; to say that it is mature is to claim that this process has reached a particular endpoint. Talking about an endpoint can make the brain seem like a finished product, no longer subject to meaningful change, but we rarely think this way about other complex systems. Would you ask when a city is “fully developed,” knowing its ecosystem is expected to keep expanding and adapting as the population that inhabits it changes? The brain should not be considered differently. However, for clinical and behavioral research —and increasingly in domains such as forensic psychology— identifying periods of relative stability provides valuable reference points for comparing development across individuals and populations. The important thing to stress is that these endpoints are analytical tools rather than objective moments at which the brain becomes complete.

The problem is that there is no single measurement by which neuroscientists determine whether a brain has reached maturity. Unlike height or body weight, brain maturation cannot be reduced to the single event of tissue growth. It refers instead to a collection of developmental processes, each progressing at its own pace and describing a different aspect of the brain. Structural development is only one dimension of this process. Researchers also study changes in functional connectivity, neurotransmitter systems, receptor density, metabolic activity, hormonal regulation, and the balance between excitatory and inhibitory activity.

Even if researchers agree to focus only on structural development, when should they say a developmental process has reached maturity? One possible criterion is the point at which a biological measure begins to plateau, meaning its rate of change slows until year to year differences become minimal. In practice, this point is operationalized as a developmental asymptote: a value derived by fitting a mathematical model to the observed developmental trajectory and calculating the age at which the curve is projected to reach a specified percentage of that value. This approach, however, is also subject to ambiguity. Different biological measures reach their asymptotes at different ages, and studies define those asymptotes using different statistical criteria. The age of brain maturity depends both on what researchers measure and on how they operationalize the endpoint of development.

The first claim assumes that brain maturity is a single biological property, but the way neuroscientists research it is through a collection of developmental processes in which each describes a different aspect of the brain. Treating them as though they converge on one objective state of maturity is, at best, an oversimplification.

Claim #2: The prefrontal cortex reaches that state by age twenty five.

I am sorry for hitting you with the same old “it depends”. Unsatisfying as it may be, many scientific disagreements begin as conceptual ones, and those conceptual ambiguities shape how empirical findings are interpreted. Whether the statement “the prefrontal cortex fully develops by twenty five” is true or false depends entirely on what fully developed is taken to mean. The problem is that this slogan rarely comes with such qualifications, as it spreads through social media as a defined neuroscientific fact. Most people using the phrase are not referring to a particular biological measure, nor to a specific operational definition of maturity; they are referring to the prefrontal cortex as a whole. Since Claim #1 showed that neuroscience provides no single biological property corresponding to brain maturity, this broader interpretation is false.

That said, I would rather describe the slogan as a misunderstanding than a lie. Certain measures of prefrontal development do reach estimated developmental asymptotes around the mid twenties. As journalist Jane C. Hu argues, the slogan appears to be the product of what she calls “pop culture telephone.” Early neuroimaging studies showed that certain structural features of the prefrontal cortex continue changing into the twenties, and that some measures approach developmental plateaus around the mid twenties. As these findings spread through popular discourse the nuance naturally disappears. Ironically, many of the neuroscientists whose work helped shape this literature acknowledge that there is nothing special about the number twenty five.

Far from the twenty five milestone, longitudinal MRI studies by Østby et al. and later by Tamnes et al. identified the prefrontal cortex as one of the latest cortical regions to mature. They did so by examining structural features like cortical thickness, surface area, gray and white matter volume, and the microstructure of white matter tracts. Across adolescence, prefrontal gray matter generally becomes thinner while the underlying white matter continues to increase because of ongoing synaptic pruning and myelination. According to these studies, the medial orbitofrontal cortex, cortical thickness is estimated to reach 90% of its developmental asymptote at approximately 39.2 years, while the rostral middle frontal cortex reaches the same point at around 31.3 years. Measures of white matter microstructure also extend beyont the mid twenties timestamps, with mean diffusivity reaching 90% of its estimated asymptote at approximately 37.4 years in the medial orbitofrontal cortex and 33.3 years in the rostral middle frontal cortex. White matter volume may well continue increasing into the fourth or even fifth decade of life.

Claim #3: Because the prefrontal cortex is taken to underlie judgment, emotional regulation, personality, and social behavior, greater biological development of the prefrontal cortex corresponds to greater performance in these domains, and conversely.

I will not scrutinize the roles associated with the prefrontal cortex presented in the claim, as its importance for executive functions, cognitive control, and related domains is well established. The interesting part is the inference that supposedly follows: that a more developed prefrontal cortex necessarily produces better performance in the behaviors and capacities associated with that region. Although intuitively appealing, this conclusion rests on assumptions rather than realities. It assumes that biological development maps directly onto behavioral performance, that these behaviors can be attributed to the maturation of a single brain region, and that greater biological development translates proportionally into better behavioral performance.

To begin with, brain development and behavior are related, but they are not the same kind of phenomenon. Development describes biological change, whereas behavior describes what an organism does under particular circumstances. Showing that a brain region continues to develop does not establish how behavior changes alongside it. Whether, and to what extent, neural development is reflected in behavior depends on many interacting influences beyond the brain, which include experience, nutrition, innate sensibilities, and so on. Brain development is one —fundamental— contributor to behavior, but not a complete account for it. This becomes clearer if we consider how new abilities are acquired. It is foolish to expect waking up tomorrow fluent in Chinese or capable of writing a great novel simply because our brains have become more developed overnight. Such abilities emerge through processes that gradually reshape neural circuitry over time. Brain development provides the biological substrate that makes these changes possible, but it is the interaction with experience that gives rise to the underlying neural organization required for new capacities to emerge. Behaviors such as better judgment should be understood much in the same way: a capacity cultivated through experience, with neural development creating the possibility for that learning to occur, not guaranteeing its outcome. Furthermore, there is rarely a one-to-one correspondence between a particular brain region and a particular psychological capacity.

Finally, even when developmental measures are associated with behavioral performance, the relationship is not necessarily proportional. For example, structural changes in cortical thickness do not imply a corresponding increase in self control or emotional maturity of the same magnitude. These relationships vary substantially across individuals and across contexts. Many, if not most, adolescents behave in ways we normally deem immature. Conversely, many adolescents display great judgment and emotional regulation in everyday life despite their brains continuing to develop. If biological maturation mapped directly and proportionally onto behavior, this phenomenon would be difficult to explain. The evidence suggests that neural development shapes the conditions under which behavior emerges without determining exactly how that behavior will be expressed.

This quote by professor Dr. Leah Sommerville sums up quite well what I have been trying to say:

“Neural activity intensifies and reduces, varies quantitatively and qualitatively, in linear and nonlinear ways that are both linked to—and independent of—behavioral differences across development.”

Or, in other words, neuroscience is hard. The relationship between neural development and behavior exists, but it is rarely simple enough to justify the kinds of straightforward causal claims that infiltrate popular discourse [2].

Claim #4: Reaching the developmental endpoint of the prefrontal cortex marks the transition into adult psychological maturity.

By this point, I hope I have made it clear beyond doubt that the answer is no. Whatever we choose to mean by adult psychological maturity, there is little reason to think that it begins at and because of the developmental endpoint of a single brain region.

The trap

In a TikTok video, half joking and half teaching, Mikhaila Peterson claimed that throughout our youth we are “literally missing a part of our brain” (one might expect more from someone with a background in biomedical science). Social media is full of videos like hers, some innocent, some making bolder claims, but none of them individually alarming. So why do I care so much? My guess is that behind every video of this sort, I smell the seductive but dangerous narrative of neurocentrism infiltrating our popular understanding of psychological phenomena.

I take neurocentrism, for the purposes of this essay, to be the tendency to treat neuroscientific findings as sufficient explanations of complex psychological phenomena, asking them to bear far more explanatory weight than the evidence justifies. This differs from Shaun Gallagher’s use of the term [3]. I have no reason to doubt that the brain is the biological substrate of mental life, or that every mental state depends on neural activity. My concern is with the tendency to invoke it as though it settled questions for which it often provides only partial or trivial answers. The brain is, of course, a necessary condition for every mental state. Our current understanding of the brain, however, lacks a complete explanation of those mental states and of the concepts through which we ordinarily understand them.

What happens when our ordinary understanding of mental concepts becomes intertwined with simplified neuroscientific explanations is that we begin to reinterpret ourselves through a language centered on the brain. A question concerning how to cultivate better judgment, for example, can be answered in terms of prefrontal cortex maturity, leading us to search for improvement in the brain as an organ [4] rather than in the practices through which judgment is actually acquired. Every explanatory framework carries implicit assumptions about what ultimately matters. A neurocentric framework encourages us to privilege neural descriptions over ones that could be more useful for the purposes that matter. But knowing that the prefrontal cortex contributes to judgment does not tell us why one person exercises good judgment while another does not, any more than knowing the chemical composition of a poem's paper and ink explains its meaning, let alone teaches us how to write a better one.

I believe that when the language we use to understand ourselves shifts toward purely neurocentric terms, our conception of agency turns toward a more deterministic one. It becomes tempting to think of ourselves as passengers of our own biology, waiting for our brains to mature and rewire before we can finally become the people we wish to be. And it is an incredibly alluring mindset, especially for the person who spends their life overthinking every action, scrutinizing every intention, assigning harsh moral judgments to themselves after every failure. Neurocentrism offers us something profoundly comforting: it relocates the burden of explanation. The problem no longer seems to lie in us, but in the current state of the brain. There is relief in believing that only the biological dimension, rather than the person as a whole, carries the greater share of responsibility.

Life is movement. Any narrative that disencourages us to act effectively jeopardizes the very process through which we grow into who we are. Breaking free from those narratives can allows us to channel that mysterious will that pushes towards change, a change that is necessary for becoming.

[1] Someone who communicates complex neuroscientific findings in more digestible and engaging ways for laypeople.

[2] This does not imply that causal relationships between the brain and behavior cannot be made. Lesions in the brain, neurodegeneration, and developmental disorders produce well documented behavioral impairments. My point is that normal variation in healthy brain development cannot be simply interpreted as proportional differences in behavioral outcome.

[3] For him, neurocentrism is a theoretical perspective that identifies the brain as the central and nearly exclusive mechanism for cognition. I do not find his neurocentrism to be a controversial position.

[4] Brain optimization is mostly a scam, believe me, I will write about it later.