What is the difference between lying and misleading?

A lie involves literally stating what you believe to be false, so that the liar is insincere about the semantic content of its utterance. By contrast, misleading involves implicating something that you believe to be false. The misleader is not insincere about the semantic content of its utterance, but nonetheless conveys some information that he believes to be false, through pragmatic mechanisms such as conversational implicatures, conventional implicatures, and the like.

Consider these two examples:

(1) I never had sex with Monica

(2) There is no sexual relationship between me and Monica

In statement (1), the speaker is denying that he ever had sex with Monica. By our definition, if the speaker did have sex with that woman, he would be lying. By contrast, in statement (2) the speaker is not saying that he never had sex with Monica: he only stated that currently there is no sexual relationship between them. Nonetheless, the statement can be misleading because (in some contexts) it can suggest or imply that the speaker never had a sexual relationship with Monica. In the even that this is false, the speaker would be misleading but not lying.

This distinction is philosophically significant for two reasons. First, it helps us to understand and investigate our intuitions about the distinction between literal and non-literal meaning. Second, this difference is often taken to be morally or juridically significant (for instance, a liar can be rightfully accused of perjury, whereas a misleader cannot). Whether this distinction is as a matter of fact ethically significant, however, is still a matter of philosophical debate.