Michael J Fox believes "gratitude makes optimism sustainable".
The 60-year-old actor was diagnosed with Parkinson's disease in 1991 - but that hasn't stopped him from retaining a positive mindset and an optimistic approach to life.
He shared: "For one thing, I am genuinely a happy guy. I don't have a morbid thought in my head - I don't fear death. At all.
"But as I came through that darkness, I also had an insight about my father-in-law, who had passed away and always espoused gratitude and acceptance and confidence.
"I started to notice things I was grateful for and the way other people would respond to difficulty with gratitude. I concluded that gratitude makes optimism sustainable.
"And if you don't think you have anything to be grateful for, keep looking. Because you don't just receive optimism. You can't wait for things to be great and then be grateful for that. You've got to behave in a way that promotes that."
The 'Back to the Future' star concedes that "some days are more difficult than others", as he continues to struggle with the degenerative disorder.
However, he doesn't allow Parkinson's to dictate his approach to life, and the actor actually considers himself to be "lucky".
Michael - whose foundation has helped to raise awareness for Parkinson's disease - told AARP The Magazine's December 2021/January 2022 issue: "The disease is this thing that attached to my life - it isn't the driver. And because I have assets, I have access to things others don't.
"I wouldn't begin to compare my experience to that of a working guy who gets Parkinson's and has to quit his job and find a new way to live. So, I'm really lucky."