problem with the "mental states" idea

Thinking on the significance of mental states, and how "the past doesn't matter." There are some problems with this. What about telling lies? Yesterday, for example, I stole a car. Today someone asks me if I stole it, and I reply "no." I am lying. To lie or tell the truth are both mental states, ones tied to a past event, and it's important, in society, that one's current mental state corresponds to a past event.