Nichiren Daishonin states in “On the Transmission of the Three Great Secret Laws”:
There are two types of Daimoku: the Daimoku chanted during the Former and Middle Days of the Law, and the Daimoku to be chanted during the Latter Day of the Law. Although Bodhisattvas Vasubandhu and Nagarjuna themselves chanted the Daimoku during the Former Day of the Law, they did so to practise for themselves only. During the Middle Day of the Law, Nanyue, Tiantai, and others did chant the Daimoku; however, they did it only for the sake of their own practice and not for teaching others. This type of Daimoku was practised [in order to observe one’s mind] based on the theory [elucidated in the surface meaning of the words]. Now, in the Latter Day of the Law, the Daimoku that Nichiren chants is different from that of previous ages. It is the Nam-Myoho-Renge-Kyo of the practice for oneself, and the practice for others.
题目有二意，所谓 正 像 与末法也。 正 法时，天亲菩萨、龙树菩萨虽 唱 题目，但仅止于自行。 像 法时，南岳、天台等 唱 南无 妙 法 莲 华经，乃为自行不为化他广 说。是理行之题目也。入末法，今日莲所 唱之题目异于前代，乃亘及自行化他之南无妙法莲华经也。