If it's a simulation....then the one and only thing that you can know FOR SURE is that you PERCEIVE reality to be such a way. Even if your current perception of sitting at a desk is a simulation....you still THINK that you are sitting at a desk. This is the basis of the phrase 'I think therefor I am'. You can only know for sure what you THINK about or how your perceive reality...not reality itself. And since you have proof that you think or perceive SOMETHING (even if it's not 'real reality')....you must EXIST!