presupposition

1. N-COUNT A presupposition is something that you assume to be true, especially something that you must assume is true in order to continue with what you are saying or thinking. (尤指必須的)假設; 預想; 預設[正式]

...the presupposition that human life must be sustained for as long as possible. ...必須讓人的生命得以盡可能延長的這種假設。