A linguistic system is a series of differences of sound combined with a series of differences of ideas.
It is useful to the historian, among others, to be able to see the commonest forms of different phenomena, whether phonetic, morphological or other, and how language lives, carries on and changes over time.
Henceforth, language studies were no longer directed merely towards correcting grammar.
Everyone, left to his own devices, forms an idea about what goes on in language which is very far from the truth.
The critical principle demanded an examination, for instance, of the contribution of different periods, thus to some extent embarking on historical linguistics.