 There is no guarantee that patients will believe medical truths or diagnoses squarely placed before them by qualified physicians. Denial of unpleasant truths is commonplace. Self-deception about health is a powerful phenomenon. Some people sincerely believe refusing to accept an unwanted diagnosis by maintaining a positive attitude will shield them from medical catastrophe. Mental disorders can trick one into thinking one is well, happy and insightful, rather than the manic, delusional person the psychiatrist may diagnose.