What do you believe is true even though you cannot prove it?

Given some of the heavyweights who responded to this question, I am (for once) not going to add my own 2 ¢. There are, however, a lot of interesting contributions to mull over. Some of those that stood out for me were:

• Oliver Morton's statement "as far as knowledge goes I'm a consumer, and sometimes a distributor, not a producer" rings very true for me.
  
• Paul Steinhardt comments about some physicists embracing artifacts of the mathematical models they use to descibe the universe as "fact".
  
• Fellow atheist Tor Nørretrander's statements about the importance of having faith (and his use of the word ineradicable!)
• Charles Simonyi's discussion on the "complexity inflation" prevelant in software development (a topic close to my heart).
  
• Margaret Wertheim's statement "that there will always be things we do not know".
• Esther Dyson on modern life: "It used to be that machines automated work, giving us more time to do other things. But now machines automate the production of attention-consuming information, which takes our time."
• Lee Smolin's comments on quantum theory (another topic close to my heart).
• Kai Krause's assertion that "It is about the anticipation of the moment and the memory of the moment, but not the moment."
  
• The number of conjectures about the nature of consciousness including Daniel Dennet's belief that acquiring language is a "necessary precondition for consciousness".