A moral inquiry is needed to decide why promises ought to be kept.
Dr. Jukka Kilpi's book The Ethics of Bankruptcy contains
an inquiry into theories of promising which contrasts and compares
the Humean doctrine of promises as useful conventions with the Kantian
view of autonomous agency constituting promissory obligations. Kant's
ideas are disentangled from some of his more dubious metaphysics,
while the notion of autonomy is preserved and used to support the
normative thesis of the ethics of bankruptcy.
In Kilpi's inquiry the revised Kantianism yields an original theory
of promissory autonomy, which overcomes some of the difficulties afflicting
Humean conventions. According to promissory autonomy, my promise is
a demonstration of my autonomous will which shapes the future through
the imposition and observance of moral obligations: it is an expression
of a free will restricting its own freedom. Hence, it is the autonomous
choice of an individual which is the ultimate moral power behind the
bindingness of promises.