As quoted in Serving "60 Years to Life", Newsweek Europe (12 December 2005)
“Be an optimist! It’s the only way to live.”
Source: The Heritage Universe, Summertide (1990), Chapter 13, “Summertide Minus Ten” (p. 151)
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Charles Sheffield 41
British scientist, American science fiction writer 1935–2002Related quotes
“[…] You see, you are an optimist and live on hope. I am a pessimist and live on experience.”
                                        
                                        Page 352-353. 
Stepping Westward (1965)
                                    
“Sometimes a pessimist is only an optimist with extra information.”
Source: Reflections
“A pessimist is a man who has been compelled to live with an optimist.”
The Note Book of Elbert Hubbard (1927)
“Optimists and pessimists differ only on the date of the end of the world.”
                                        
                                        p, 125 
Unkempt Thoughts (1957)
                                    
                                        
                                        This is actually James Branch Cabell from The Silver Stallion (1926) 
Misattributed
                                    
Source: On balancing pessimism and optimism in “Kamila Shamsie: 'We have to find reasons for optimism’” https://www.theguardian.com/books/2018/jun/08/kamila-shamsie-we-have-to-find-reasons-for-optimism-home-fire in The Guardian (2018 Jun 8)
Source: Your Forces and How to Use Them (1912), Chapter 6, p. 97