Unification in the Unified Ontology of Human Function: A Whole-Person Conceptual Framework Centered on Human Function and Functional Engagement

This paper develops a conceptual framework for understanding unification in the Unified Ontology of Human Function (UOHF). UOHF begins from a basic ontological premise: the human body exists and operates as an integrated whole before it is divided into the separate objects of medicine, physiology, rehabilitation, exercise science, psychology, nutrition, clinical informatics, or ontology engineering. Respiration, circulation, metabolism, sensation, cognition, movement, defense, repair, recovery, behavior, and real-life activity all occur in the same person. Molecules, cells, tissues, organs, organ systems, physiological processes, behavior, environment, and life tasks are not independent worlds, but different scales, conditions, and manifestations of the same human being. For this reason, unification in UOHF does not mean mechanically combining disciplinary vocabularies, classifications, datasets, professional methods, or knowledge systems into one database. It also does not mean prescribing an ideal way in which the body ought to operate. Unification means using one coherent semantic system to describe the human functions that already exist within the same whole person, together with the demands they serve, the bodily structures and processes through which they are realized, whether they actually operate under concrete conditions, and how coordination, compensation, boundaries, state, time, and change unfold. The central proposition of this paper is: Unification in UOHF means using one coherent semantic system to describe human functions as they exist within an integrated whole person, together with their functional engagement. Within this framework, unification includes at least six connected requirements. First, UOHF must represent what human functions exist and establish completeness rules that can continuously identify omissions. A fixed list of functions cannot be treated as permanently complete. Second, UOHF must represent which human functions are required to meet different internal and external demands. Internal demands include ongoing requirements for physiological maintenance, regulation, development, defense, adaptation, repair, and recovery. External demands include behavioral, environmental, and real-world task demands. Third, UOHF must connect human functions to the bodily structures and bodily processes through which they are realized, while preserving distinctions among whole-person human function, local biological function, bodily structure, bodily process, functional capacity, manifestation, measurement, and inference. Fourth, UOHF must represent functional engagement: whether the human functions required by a specified internal or external demand actually operate when that demand arises. Fifth, UOHF must represent the conditions under which functions operate, including coordination, compensation, cost, burden, boundary, reserve, state, time, and change. Sixth, these facts and relations must remain connected to the same person, under specified conditions, at specified times, and across continuous longitudinal change. Human function is formally defined as: Human function is the body's capacity to be appropriately engaged to meet internal and external demands. The corresponding Chinese philosophical definition is: 人体功能=身体被正常调用的能力。 Within this definition, functional engagement is not a separate ontological construction principle parallel to unification. It is an indispensable relation within the concept of human function. Function describes what capacities the body has. Functional engagement describes whether the human functions required by a concrete demand actually operate in a particular person, under specified conditions, and at a specified time. The philosophical definition of functional engagement is: Functional engagement is whether the human functions required by internal and external demands actually operate when those demands arise. Its formal meaning is the context-specific relation between a specified demand and each human function required to meet it, according to whether that function actually operates through corresponding bodily structures and bodily processes in a particular person, under specified conditions, and at a specified time. Functional engagement does not mean conscious command, voluntary participation, patient engagement, service engagement, or isolated neural or muscular activation. It refers to whether the required human functions actually enter into operation in response to a demand. This distinction makes it possible to separate two related but non-identical kinds of problem. A functional-capacity problem primarily concerns insufficiency, impairment, or loss of the required functional capacity itself. A functional-engagement problem concerns a situation in which relevant functional capacities are supported by evidence as present, but do not actually operate under a specified internal or external demand. These two kinds of problem may coexist. They must not be forcibly separated when the available evidence is insufficient. UOHF therefore distinguishes between possessing a capacity and successfully engaging that capacity. A person may possess a relevant capacity in principle but fail to engage it effectively, stably, sustainably, or appropriately under a specific demand. Conversely, failure under a demand may arise because the required capacity itself is insufficient, impaired, or absent. The distinction must be established through evidence rather than assumption. The framework also provides a shared semantic axis for cross-professional interpretation: functional capacity → restoration of appropriate engagement → maintenance and enhancement of human function and expansion of engagement boundaries Within this axis, medicine can primarily address functional capacities, their biological foundations, disease mechanisms, and safety boundaries. Rehabilitation can primarily address the return of functional capacities to real-world tasks and the restoration of appropriate engagement. Exercise can maintain and enhance human function and expand engagement boundaries through sustained, appropriately dosed, and progressive engagement. Psychology and nutrition enter when indicated by the current primary limiting factor, available evidence, and professional scope. This division does not establish absolute professional boundaries. Medicine, rehabilitation, exercise, psychology, and nutrition may overlap in practice. The purpose of the framework is to distinguish their principal functional objectives within one shared human-function semantic system. UOHF also distinguishes three levels of representation. The reality layer describes what exists and occurs in the human-function world itself, including persons, functions, demands, bodily structures, bodily processes, engagement, coordination, compensation, boundaries, states, time, and change. The epistemic layer describes how these realities are observed, reported, measured, assessed, inferred, and judged, including evidence, uncertainty, provenance, and professional interpretation. The application layer describes governed actions intended to influence reality, including medical treatment, rehabilitation, exercise, psychological intervention, nutritional support, environmental modification, assistive technology, and other professional actions. Assessment tools, measurements, interventions, and professional methods must not be used to define human function itself. They provide evidence about human function or act upon it, but they are not identical to the reality they observe or modify. The paper further argues that unification requires more than placing information in one technical system. Disease, structure, physiological process, task, performance, measurement, intervention, and professional record may all refer to the same person, but they do not automatically form a unified representation. They become unified only when their objects and relations can be consistently connected to the same human functions, the same demands, the same person, and the same longitudinal context. UOHF therefore aims to provide a conceptual foundation through which different scales and professional perspectives can remain distinct while still referring to one coherent whole-person human-function object. External classifications, clinical terminologies, physiological models, biological ontologies, assessment systems, and professional knowledge retain their original identity and scope. UOHF does not replace them. It provides a common semantic center through which their relevant content can be related to human function and functional engagement in the same person. This paper is a conceptual ontology framework and research proposal. It does not claim that a complete catalogue of human functions, all demand–function relations, all structure–function and process–function mappings, all multiscale relations, a fully axiomatized machine-readable ontology, external expert consensus, clinical validation, or an autonomous diagnostic or treatment system has already been completed. Its purpose is to establish the conceptual center, object boundaries, relation structure, completeness requirements, and formalization direction needed to describe human functions and their functional engagement within the same whole person, under internal and external demands, across specified conditions and continuous time.