The work argues that mental energy is a finite, physiological resource that should be gathered and conserved rather than squandered. Drawing on experimental psychology, neurology, and physiology, it explains how physical and mental fatigue interrelate, how emotions and drugs alter consciousness, and how brain regions and neurons localize perceptions, memory traces, and capacities. It examines learning, talent, and vocational suitability, warning that pursuing tasks mismatched to one’s neural makeup or indulging destructive passions wastes psychic strength. Practical implications for education and self-care are considered, including how overfilling neuronal capacities eventually limits new learning and weakens adaptation to life’s demands.