presupposition
US /ˌpriː.sʌp.əˈzɪʃ.ən/
UK /ˌpriː.sʌp.əˈzɪʃ.ən/
1.
a thing tacitly assumed beforehand at the beginning of a line of argument or course of action
:
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The theory is based on the presupposition that all humans are rational.
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His argument carries the presupposition that we all agree on the definition of success.