A comparison is given between two conditions used to define logical constants: Belnap's uniqueness and Hacking's deducibility of identicals. It is shown that, in spite of some surface similarities, there is a deep difference between them. On the one hand, deducibility of identicals turns out to be a weaker and less demanding condition than uniqueness. On the other hand, deducibility of identicals is shown to be more faithful to the inferentialist perspective, permitting definition of genuinely proof-theoretical concepts. This kind of analysis is driven by exploiting the Curry-Howard correspondence. In particular, deducibility of identicals is shown to correspond to the computational property of eta expansion, which is essential in the characterization of propositional identity.1 Note also that Belnap requires the deducibility relation to satisfy a set of structural properties that contains the cut rule and the weakening rule(s); however, unlike Hacking, he does not explicitly ask for the admissibility of these rules (cf. Belnap, 1962, p. 132).3 The difference between uniqueness and strict uniqueness (Došen and Schroeder-Heister, 1985, p. 166) is not taken into account here: in a sequent calculus setting such a difference is difficult to identify in a very neat way. 4 Belnap's original definition of UNI is slightly different from the one we presented here. However, in presence of transitivity of the deducibility relation -i.e., in presence of the cut rule -our own definition entails Belnap's one. The latter will be discussed in more details in section 5.2.