Deictically...
I recently came across this cartoon while reading The New Yorker online. It depicts a woman going past a bar where someone has taped a sign in the window that says ‘I’m not here’.
Part of the humour here, of course, is that whoever put the sign up is probably in the bar, making the sign untrue. In fact, the statement ‘I’m not here’ can actually never be true.
I and here are deictics, meaning that what they refer to changes according to the situation in which they are used. The pronoun I represents something like ‘the person who is speaking’, and here represents something like ‘the location of the person who is speaking’. Saying ‘I’m not here’ is thus equivalent to saying something like ‘The person who is speaking is not located at the location of the person who is speaking’. In contrast ‘I am here’ will always be true (‘The person who is speaking is located at the location of the person who is speaking’), along with ‘I’m not there’ (‘The person who is speaking is not located at a location that is not the location of the person who is speaking’).
Of course, the ‘I’m not here’ in the cartoon is written, not spoken. So while it was true at the time of writing, it may not necessarily be true later on. For instance, the person who wrote the sign could have gone home by the time the woman sees it. It turns out there is actually a third deictic element in the sentence: tense. The interpretation of ‘I’m not here’ thus becomes the rather complicated ‘The person who is speaking/writing is not located at the location of the person who is speaking/writing at the time the person who is speaking/writing is speaking/writing.’ In contrast, ‘I wasn’t here’ can be a perfectly truthful statement. In other words, ‘It depends on what the meaning of the word “is” is.’
As a last point, there are other interpretations of here. For instance, speaker might use here when gesturing to another location (for example, pointing to a place on a map). However, other meanings of here don’t apply in this situation.
