While substitutions between some pairs of words like car / automobile or water / H 2 O are truth preserving, the words are still not identical in meaning. Indeed, probably no two words are absolutely identical in meaning. One of the fundamental tenets of semantics, called the principle of contrast (Girard 1718, Bréal 1897, Clark principle of contrast 1987), states that a difference in linguistic form is always associated with some difference in meaning. For example, the word H 2 O is used in scientific contexts and would be inappropriate in a hiking guide-water would be more appropriate-and this genre difference is part of the meaning of the word. In practice, the word synonym is therefore used to describe a relationship of approximate or rough synonymy.