The book discusses the central notion of logic: the concept of logical
consequence. It shows that the classical definition of consequence as
truth preservation in all models must be restricted to all admissible
models. The challenge for the philosophy of logic is therefore to
supplement the definition with a criterion for admissible models. The
problem of logical constants, so prominent in the current debate,
constitutes but a special case of this much more general demarcation
problem. The book explores the various dimensions of the problem of
admissible models and argues that standard responses are unwarranted. As
a result, it develops a new vision of logic, suggesting in particular
that logic is deeply imbued with metaphysics.