Western Buddhists often believe and proclaim that metaphysical speculation is irrelevant to Buddhist ethics or practice. This view is problematic even with respect to early Buddhism, and cannot be sustained regarding later Indian Buddhists. In Śāntideva’s famous Bodhicaryāvatāra, multiple claims about the nature of reality are premises for conclusions about how human beings should act; that is, metaphysics logically entails ethics for Śāntideva, as it does for many Western philosophers. This article explores four key arguments that Śāntideva makes from metaphysics to ethics: actions are determined by their causes, and therefore we should not get angry; the body is reducible to its component parts, and therefore we should neither protect it nor lust after other bodies; the self is an illusion, and therefore we should be altruistic; all phenomena are empty, and therefore we should not be attached to them. The exploration of these arguments together shows us why metaphysical claims can matter a great deal for Buddhist ethics, practice and liberation.