Reconceptualising Mental Fatigue via Perceptual Control: From Concept Label to Constitutive Definition

Mental fatigue is widely invoked to explain performance decrements across high-stakes domains. The prevailing definition calls mental fatigue "a psychobiological state caused by prolonged demanding cognitive activity and characterised by subjective feelings of tiredness." This definition has two flaws. It confuses the label with the constitutive properties of the concept, and it treats the dissociations among self-report, performance and physiology as mixed measurement effects, when those dissociations are diagnostically informative. We argue this conflation reflects a definitional problem disguised as a measurement issue. Drawing on perceptual control theory, we reconceptualise mental fatigue as a transient state of an effort-regulation control system. In this state, the system's controlled perception-functional viability-persistently remains below its reference during sustained cognitive control. This state arises from, and is maintained by, resource-goal conflict-conflict between resource-management systems (signalling conservation needs) and goal-management systems (signalling task importance). Our constitutive definition does four things. It clarifies the entity (a specific regulator), specifies the property (persistent error reflecting unresolved resource-goal conflict) and articulates the intrinsic attribute (viability below reference). From these clarifications, it derives observable indicators along an economisation continuum-efficiency adjustments, reduced precision, elevated effort, disengagement-without collapsing the concept into any single indicator. Neighbouring constructs are distinguished by their conflict structure and economisation profile. The framework is offered as a falsifiable architecture that organises measurement, generates testable predictions, and accelerates the development of credible psychobiological markers and countermeasures.