After an overview of the psychiatric literature on the psychopathology of "as if" or that is to say "Als ob Erlebnis", we made a study of one obsessive-compulsive patient from the view point of "as if". Through this approach, we identified a specific experience-field, namely the appearance, what seems to a person such and such. The psychopathology of "as if" could be linked closely to the appearance. Without the possibility of error, one cannot recognize the reality of being in the world and being accessible to anybody. In contrast to this, one escapes any errors with regard to the appearance because the appearance relates to nobody but the person to whom it appears. So, concerning the appearance that seems to a person such and such, he/she is subjectively certain and not to be corrected by others, just like a delusional patient. In daily life, we are often unaware that there are confusions between reality and appearance. The obsessive patient cannot permit themselves such customary confusions. The delusional patient cannot realize irrelevant confusions of reality and appearance in spite of encounters with other persons. To elucidate the delusion, we used a similar analytical way to that introduced by Spitzer. In spite of this, we arrived at a different consequence. We discussed such a discrepancy emerging and mentioned some problems associated with the delusion by investigating the uniqueness of the appearance.