Original sentence: Asia is a vague concept as is presently used.Asia is either regarded as a landmass of its own, or as part of Eurasia.It is a historical concept with European roots.One conception of Asia is defined by subtracting Europe from the more scientifically proper continent, Eurasia
Edited sentence: Asia is a historical concept with European roots.Defined by subtracting Europe from the more scientifically proper continent, Eurasia, Asia is either regarded as a landmass of its own, or as part of Eurasia