In historical practice, some historians apply counterfactual method to inquire something possible in history. M.Heidegger also had a similar idea. He said, the theme of historiography is neither that which has happened just once for all nor something universal that floats above it, but the possibility which has been factically existent. These two points are not to inquire something which hasn't happened but to reset the subject-matter, quality and method of historical writing. However, it is very different between the possibility of counterfactual studies which is contingency of something present-at-hand and the possibility as an existentiale which is existentially possibility of Dasein.