This paper introduces valuation structures associated with preferential models. Based on KLM valuation structures, we present a canonical approach to obtain injective preferential models for any preferential relation satisfying the property INJ, and give uniform proofs of representation theorems for injective preferential relations appeared in the literature. In particular, we show that, in any propositional language (finite or infinite), a preferential inference relation satisfies INJ if and only if it can be represented by a standard preferential model. This conclusion generalizes the result obtained by Freund. In addition, we prove that, when the language is finite, our framework is sufficient to establish a representation theorem for any injective relation.

Ab&&For the semantic characteristic problems of Medium hposilion Logic (MP), some people had given out the three values interpretation (model) in the past, so many scholprs (include founders) affirmed that medium logic is one kind of three valued logic. But to a r m MP is a three valued lo&, it is neee5881y to prove that MP does not edst other interpretations, for example, the intuitionist proposition logic is a dassieal model but is not a classical logic An interpretation of infinite valued of M p bas been given, and concepts which dehed the infinite (or bite) valued valid formula, theorems of completeness and reliability have been proved. Themfore we can show conclusion that ''the medium propmition logic is one kind of three valued logi~?' is incorrect.

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