Since the 19th century, fatigue has received far more attention in the medical literature than feeling of fatigue. The latter is defined as a primary, sui generis feeling, which can be studied (and is experienced) independently of physical phenomena such as tiredness, and cognitive concomitants such as "not wanting to continue or initiate a task." The unexplained feeling of fatigue is a common medical complaint, and accompanies various medical and psychiatric conditions. Several meaning systems can be identified in relation to feeling of fatigue and provide material for construction of a measuring instrument. Such a scale will allow us to answer four questions: Is the feeling of fatigue a unitary and primary experience or sensation? Is it the same regardless of the clinical or social context in which it occurs? Are the somatic and cognitive accompaniments of feeling of fatigue an essential part of its definition? Does feeling of fatigue have a neurobiological basis?