So first off existence vs what is the reality of existence is sorta two different thing. Descartes pretty much sums it up there which is kind of funny as it did not really click when I was younger but I finally sorta got it. My reality starts with my thoughts. Having thoughts means I exist in terms of if existence is a thing. If my existence is false my concept of existence is false and therefore I have a false existence that I see as existence which is a type of existence. From there it gets a bit more difficult but follows sorta the same. With your thoughts you have perceptions. They could be true or they could be false but from your perspective your perceptions come at you relatively consistently with your actions so either way they functionally are a type of existence. When it comes to others they appear to have knowledge I do not and I learn things from them and the world. So there is some independent existence to them. There is a possibility of a false existence that is completely encompassed by you but if that is the case then you apparently hide knowledge from yourself till it fits the story. Once again regardless the nature of the existence you perceive and interact with works with the best consistency if you interact with people as if they are their own independent existent beings. So for all practical purposes they exist. As you can see it pretty much continues on with WYSIWYG. What you see (experience) is what you get.