As "Bushosho" by Tokuitsu was the book of critique of Hokekyo, which could not be ignored by Saicho, he had to fiercely oppose this opponent to secure the raison d'etre for his own sect.
徳一の『仏性抄』は最澄にとって看過しがたい法華経批判の書であり、この法敵に猛反論しなければ自宗の存在意義そのものが危うくなる。