The paper provides a method for a uniform complete Hilbert-style axiomatisation of Post's -conditionals and Post's negation, where m is the number of truth values and u is the number of designated truth values . The main feature of the technique which we employ in this proof generalises the well-known Kalmár Lemma which was used by its author in his completeness argument for the ordinary, two-valued logic.

. The connection presented in this paper mirror-links two metamathematical structures, the finitary closure operators, and the compact consistency properties, in such a way that a specification of one structure induces a provably equivalent specification of the other.

The method based on the idea of Galois connection is well known. It facilitates investigations into similarities between mathematical structures, including isomorphisms between these structures, the highest degree of similarity. This idea is employed here and adapted so as to get to the core of aspects of the relationship between some metamathematical structures. The focus is put on the relation between traditional methodological orthodoxy based on the idea of proof , on the one hand, and on some alternative methodological set-ups (...) based on other ideas such as consistency or some forms of maximality, on the other hand. (shrink)

By indirect-deduction theorems introduced in the present paper we mean the theorems that allow to formalize indirect reasonings occurring in deductive practice in general and in mathematics in particular. We discuss the relationship between the introduced theorems and some logical calculi being virtually confined to propositional calculi with implication and negation. It is worth to notice that the above theorems are very handy and effective in proving logical theses.

the standard metalogical set-ups seem to be all based on the idea of consequence (or proof). However, metalogic can also be effectively constructed using some non-standard primmitive ideas. In this paper an outline is given to four metalogical frameworks, alternative to the standard set-ups. They are based, respectively, on the idea of consistency; on an omission (or separation) operator; on an extension operator (called in the paper a Lindenbaum operator); and on the idea of maximality. All these metalogics, including the (...) standard consequence-based one, are one-to-one and proof-preserving translatable, and, in this sense, are seen pairwise equivalent and „externally” consistent. (shrink)