“To experience the loneliness of soul is the hardest thing in the world.”
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Brigitte Bardot 35
French model, actor, singer and animal rights activist 1934Related quotes

“Communication, hardest thing in the world.”
Jim Carrey's Unnatural Act (1991)
Context: Communication, hardest thing in the world. Y'know, I can look at you guys, I can communicate to you all night, but, one-on-one, I'm terrible. It's just, there's certain things about communicating that really bother me. Like whenever I meet somebody new I say, "Hi! How are you!" Most of the time when people hear that they'll say, "Good! And yourself?", or "Fine! Thank you very much!" But sometimes they like to surprise you, "I've got no dream, man! I'm all dead inside!"

“The hardest thing in the world to understand is income taxes.”
Attributed by his friend Leo Mattersdorf, who also said that "From the time Professor Einstein came to this country until his death, I prepared his income tax returns and advised him on his tax problems." In a letter to Time magazine, 22 February 1963. See this post from The Quote Investigator http://quoteinvestigator.com/2011/03/07/einstein-income-taxes/#more-2031 for more background.
Attributed in posthumous publications

“It's the hardest thing in the world to go on being aware of someone else's pain.”

As quoted in Quit Your Day Job!: How to Sleep Late, Do What You Enjoy, and Make a Ton of ... (2004) by James D. Denney, p. 124 https://books.google.com/books?isbn=1884956041

“Tis the hardest thing in the world to be a good Thinker, without being a strong Self-Examiner.”
Vol. 1, p. 92; "Soliloquy: or Advice to an Author".
Characteristicks of Men, Manners, Opinions, Times (1711)

Don Soderquist “ Live Learn Lead to Make a Difference https://books.google.com/books?id=s0q7mZf9oDkC&lpg=pg=PP1&dq=Don%20Soderquist&pg=PP1#v=onepage&q&f=false, Thomas Nelson, April 2006 p. 87.
On Listening