I only know I am a thinking thing. My senses can be deceived, I could be a brain in a vat being fed chemical stimulation. 
I consider myself to be made of the same stuff you might be made of, I perceive you acting autonomously. So I will respect it and aim for the best moral utility of each of my actions. I can't prove you wrong any more than you can prove me right. I consider us individuals tring to act as morally as we can in every situation (call us souls or whatever lexicon you wish)