I'm very much divided on this subject because there is a big difference between what is classified as a "Neurotic depression" and a "Psychotic depresssion"
Basically, when suffering from Neurotic depression, we do not suffer from irrational thinking - but a psychotic depression will cause this problem - eg, irrational thinking, delusions, paranoia etc.... I have definitely seen this with my son and it's very, very unpleasant. He had no hope of doing anything at all because he was so very agitated. (I've been on the cusp of this before, so could see what was happening to him but I always knew that I wasn't quite "right" and thereby lies the difference.)
He had absolutely no idea that he wasn't thinking in a "normal way." To be honest - "normal" didn't even come into it - but if you had spent just 5 minutes with him, you would not know that he was severley ill... I knew though and so did my husband and daughter but it took ages to get the correct diagnosis.
I'm not trying to say that depression is any less debilitating in any form but a serious mental illness such as schizophrenia or Bi Polar Disorder is incredibly serious and the medication can have a serious impact on the patient's way of life... (If it works..)
http://maddieruud.hubpages.com/hub/Psychosis_Vs_Neurosis - I think this website explains it very well indeed....
Valxx (Sorry to go on so much!)
Valxx