I created all of humanity by myself, and when I interact with it, I am in fact only interacting with myself again, because there is no one else.

 It’s me who created humans. Because they are not things that exist on their own; they are things that exist in my mind. Simply put, if the thing that allows me to perceive them were to disappear, they too might “disappear.” So if I did not have the consciousness that makes it possible for me to recognize humans, they would vanish. This means I created all of humanity by myself, and when I interact with it, I am in fact only interacting with myself again, because there is no one else.