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. (尤指必须的)假设; 预想; 预设[正式]

...the presupposition that human life must be sustained for as long as possible. ...必须让人的生命得以尽可能延长的这种假设。