  Accounting for the epistemic contribution of deduction has been a pervasive
problem for logicians interested in deduction, such as, among others, Jakko
Hintikka. The problem arises because the conclusion validly deduced from a set
of premises is said to be "contained" in that set; because of this containment
relation, the conclusion would be known from the moment the premises are known.
Assuming this, it is problematic to explain how we can gain knowledge by
deducing a logical consequence implied in a set of known premises. To address
this problem, we offer an alternative account of the epistemic contribution of
deduction as the process required to deduce a conclusion or a theorem,
understanding such process not only in terms of the number of steps in the
derivation but, more importantly, the reason for or justification for every
step. That is, we do not know a proposition unless we have a justification or
proof to hold that proposition. With this goal in mind, we develop a
justification logic system which exhibits the epistemic contribution of a
deductive derivation as the resulting justified formula.
