When you think about it the only way to be authentic is to be completely present in the here and now. Only then your behavior will be completely uncontaminated by thoughts about either present or past. Just reflect on the fact how your thoughts are always with you. You might think of holding your belly in because somebody wants to take picture of you and you remember how embarrassed you felt when you where caught off guard a while ago or you might be frowning because you are solving a problem in your head.