The quality or state of being capable of making mistakes or being incorrect; fallibility.
From the adjective 'fallible' plus the abstract noun suffix '-ness'. This is a near-synonym to 'fallibility' but with slightly more emphasis on the concrete state or condition.
While 'fallibility' is the philosophical concept, 'fallibleness' sounds more personal and concrete—it's the word you'd use to describe the *feeling* of knowing you might be wrong right now, which is basically the human condition philosophers call 'anxiety.'
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