For the reason of their variation F. de Saussure excluded 'sentences' from the linguistic study. Certainly, the forms of sentence, which contains from one-word sentences to multiple complex sentences, are almost infinitely different. Then, why do we call them all 'sentence'? What is the condition to call them 'sentence'? This work shows the reason why we recognize a group of sounds as a sentence. Though the phases of the moon are various from a full moon to a new moon, we all call them 'Moon'. We see the same reason in sentences and conclude what a sentence is after the examination of the...