“The importance of the theory of {{w|differential invariant}}s in physical problems is due to the fact that when coordinates are used to describe physical phenomena (e.g. those studied in geometry) it is usually the case that the coordinates are no part of the phenomena themselves. They are generally put into the description by the observer. Therefore it is desirable to have the description in such terms that when stated in terms of one coordinate system, it can be read off easily in terms of other systems. Such statements will employ invariants of one sort or another.”