In addition to discussing some philosophical accounts of common sense, this article considers several ways in which common sense can be altered or disturbed in psychopathology. Common sense can be defined as practical understanding, capacity to see and take things in their right light, sound judgment, or ordinary mental capacity. The philosopher Vico described it as the ability to distinguish the probable from the improbable. Goethe understood common sense as an "organ" that is formed in communication for the purpose of (...) communication. Common sense is rooted in the intersubjective constitution of the life-world. Obsessive-compulsive patients show relatively mild disturbances of common sense. Patients with major affective psychoses, in their premorbid personality structure, are overly attached to common sense. In schizophrenia, however, there seems to be a true abdication of common sense involving a loss of "natural self-evidence." Even in premorbid states, such persons often lose both the sense of tact and the ability to "take things in their right light." Although logic, and the ability to engage in theoretical discussion, may be preserved, there is a loss of interpretive skills and the "faculty of judgment"; this results in an inability to cope with everyday practical and social activity. There is also a characteristic sense of perplexity, a sense of amazement before that which would normally seem self-evident and a frequent tendency to reflect upon the conditions of possibility of existence that otherwise remain concealed. The author suggests that the fragility of common sense should not be seen as a mere deficiency state. Rather, it derives from a basic vulnerability inherent in the very structure of being human. (Abstract written by special issue editor: L.A.S.). (shrink)