It is stated in the Talmud (Kiddushin 82b): "Rava said: If one drinks wine, he has fulfilled his obligation; if one drinks four cups of wine, he has not fulfilled his obligation of freedom." Rashi explains: "If one drinks wine, he has fulfilled his obligation – meaning, if he drinks four cups. If one drinks four cups of wine, he has not fulfilled his obligation of freedom – in other words, it is not a complete fulfillment of the mitzvah." The Talmud continues: "If one drinks them all at once, he has fulfilled his obligation with wine; if he drinks them in four cups, he has not fulfilled his obligation." Rashi explains: "If he drinks them all at once, it is due to the joy of the holiday. If he drinks them in four cups, each cup is considered like the first cup, and he must drink three more cups in the proper order." It is evident that the wine is intended for the joy of the holiday, which is related to freedom, and therefore it must be diluted. This approach is similar to the opinion of the Tosafot mentioned later on.