An inference can be seen as a speech act, in which one passes from a number of assertions called premisses to another assertion, the conclusion, which is presented as supported or justified by the premisses. To justify the assertion that appears as conclusion is the characteristic aim of an inference. Here, we confine ourselves to deductive inferences where the justification is taken to be conclusive. A short, natural explanation of what it is for a (deductive) inference to be valid is (...) to say that it is valid if it succeeds in its aim, that is, if its premisses do (conclusively) justify the conclusion, given that premisses are justified. The paper is concerned with developing this explanation further. (shrink)