The author analyses five complex sentences of causal-semantic type in order to distinguish between an argument and an explanation in such structurally similar complex sentences. In accord with American philosopher G. R. Mayes (2000), he recognizes differences between them. The presence of epistemic modality in the main clause of a complex sentence can be seen as one of the basic distinctive linguistic signals that differentiates an argument from an explanation. From the pragmatic point of view, an explanation describes causal relations in the real world (speech sphere included), as opposed to an argument, which makes acceptable 'causal relations' in the world of speech. The hierarchy of the argument and the explanation is also the object of scrutiny. Here an argument is understood as a superstructural, additional function that a sentential segment (e.g. an explanation as well) can receive in discourse for various strategic reasons. Finally, the author analyses an eventual correlation of the distinction between an argument and an explanation and inductive and deductive reasoning.