How many times have you thought to yourself that you KNOW for a FACT that you were told something only for that person to blatantly lie? With a straight face they say they never said that! No matter what "proof" you may have the other person stands by their assertion that they never said what you are claiming.
This is gaslighting. This is all done in an effort for you to start questioning your own sanity. "Did they really say that or was I dreaming and this wasn't actually said?" When this happens a person may start believing the lies that the gaslighter is saying. Your reality starts to crumble and now you don't know if what you believe is true.
Has this ever happened to you?