The paper concerns a central issue of alethic modality. It attempts to provide a criterion of meaning for statements with modal words: ?necessary?, ?possibly?, ?must?, ?can?, ?could?, etc. By considering the main problems concerning modal logic, logic dealing specifically with modal language, the paper chooses to understand the meanings of modal language by ?possible worlds? semantics, and tries to make it more credible by employing a concept of causality which underline most of our normal modal language. Furthermore, the paper attempts to answer the following questions: Why are formal expositions essential to philosophically understanding problematic modal discourses?; What is the conceptual burden they impose on us which needs to be overcome?; Why are existing philosophical endeavours taking part in this semantic contest ?unsatisfactory? or ?inadequate??; How can we understand modal discourses causally?