Robert has had major depressive disorder for most of his adult life. as he became older, he would tell his wife that when he uses the kitchen sink, he believes his corpuscles are being washed out of his body. robert is exhibiting

The answer is a delusion. 
Robert is exhibiting delusion due to his depressive disorder. A delusion is a kind or characteristically a symptom of mental disorder which made a person having a distinctive belief or impression that is positively maintained despite being opposed by what is generally accepted as reality or logical argument just like what Robert portrays.

Answer/// absolute and unchanging laws;