大乘起信論裂網疏 6.107

6.107

此總明依四諦境，發四弘誓。上求佛道名決定智，下化眾生名廣大悲，弘能任重名大勇猛，毅能道遠名大誓願也。

This generally clarifies giving rise to the four great vows based on the realm of the four truths. Seeking the Buddha's path above is called decisive wisdom; converting sentient beings below is called vast compassion; being vast and able to bear heavy burdens is called great courage; being steadfast and capable of a long journey is called great vows.