Clouding of consciousness, also known as mental fog, is an abnormality in consciousness. The sufferer experiences a subjective sensation of mental clouding described as feeling “foggy” being in a “dreamy state” or feeling “out of it”. Pathophysiologically, it is believed to be a manifestation of an abnormality in the regulation of the “overall level” of cortical function, referred to by neurologists as “arousal”. Thus, some authors prefer the more objective term “abnormal level” of consciousness over the subjective term “clouding” of consciousness. In the 1817 German treatise Verdunkelung des Bewusstseins, Greiner first coined and pioneered the term clouding of consciousness as the main pathophysiological feature of delirium. It is poorly recognized and poorly researched by conventional doctors, who tend to mislabel it or “psychologize” it. Alternative medicine practitioners popularly use the term “brain fog”; however there is no mention as to whether they intend the term to be synonymous with the conventional medicine term clouding of consciousness.