  “[B]y superficies we do not here mean any portion of the surrounding body, but merely the extremity which is between the surrounded body and that surrounded, which is but a mode; or […] we mean the common surface which is a surface that is not a part of one body rather than the other, and that is always considered the same, so long as it retains the same magnitude and figure.” [René Descartes, Principles of Philosophy, Part II, Principle 15 (1911: 261)]