The best replacement for certainty is doubt.

The best alternatives to certainty are rated as following:
★★★★★  doubt
★★★★  uncertainty
★★★★  provisionalism
★★  certitude
★★  severity
★★  probability
★  doubt spanish
  depends
  decision making under uncertainty
  possibility
  knowledge
  decision making under uncertainty why

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Speaking about certainty, one probably shouldn't forget about proof.

Other search terms related to certainty and their ratings are:
★★★★★ proof
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