Certainty is the state of being completely sure that something is true or will happen. It is the opposite of doubt or uncertainty.
From Old French 'certeinte', from Latin 'certus' meaning 'sure, fixed, settled'. The word has always carried the idea of something decided and reliable.
The root of 'certainty' is the same as in 'certify' and even 'certificate'—all about making something officially sure. In a way, your brain is constantly issuing tiny internal certificates saying, 'Yes, I’m sure about this.'
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