Bodhicitta is the hinge of the Mahayana: the point at which a practitioner stops working on their own liberation as the goal and takes on everyone's. Shantideva's eighth-century manual treats its arising as the great event of a life — the moment a person becomes, in his phrase, a child of the buddhas — and gives a methodical training for producing it, since it is not expected to appear on its own. The relative form is the aspiration and the conduct that follows; the ultimate form is the direct realisation of emptiness that keeps the aspiration from being sentimental. The vow attached is knowingly impossible, and the impossibility is load-bearing: a goal you could finish would let you stop.