Final answer:
The original idea that dreams are significant in understanding mental illness can be attributed to Sigmund Freud, who used dream analysis as a window into the unconscious mind.
Step-by-step explanation:
To credit the original idea that dreams play an essential role in understanding mental illness, we must look to Sigmund Freud. He was a pioneering Austrian neurologist who implicated the unconscious mind as a significant factor in human behavior and mental health.
His theories emphasized that dreams could provide important insights into the unconscious mind, revealing the thoughts and feelings that we are not aware of during our waking lives. Although Carl Jung also focused on the content of dreams, it is Freud's work, particularly his method of dream analysis, that established the foundation for the view that dreams can reveal deeper psychological truths.