Decartes once said "cogito ergo sum" or simply "I think, therefore I am." Though it is true that your observations can be altered and your judgment deceived, it can be concluded (and was by Decartes) that my mind and my own thoughts exist because I am thinking and if my mind were not to exist, then I couldn't be able to have the thoughts that resulted from my deception so I have to exist.
tl;dr version, it's true that I exist because I think.
Aha but in my mind I have thought of the reality in which you do not exist. And my thoughts exists just as much as you do therefore you don't exist and neither does my thought.
Decartes once said "cogito ergo sum" or simply "I think, therefore I am." Though it is true that your observations can be altered and your judgment deceived, it can be concluded (and was by Decartes) that my mind and my own thoughts exist because I am thinking and if my mind were not to exist, then I couldn't be able to have the thoughts that resulted from my deception so I have to exist.
tl;dr version, it's true that I exist because I think.
Aha but in my mind I have thought of the reality in which you do not exist. And my thoughts exists just as much as you do therefore you don't exist and neither does my thought.