Excepting playschool stuff, I'm ignorant of the philosophical context, so it's likely I've been anticipated / in anticipation refuted. Nevertheless—
We cannot know there is something outside ourselves. Yet the concept of an 'outside' to self derives from the very illusion which, if there is nothing outside ourselves, disguises this nothing. Does the concept of a self derive so too? Does something which is not the self invent or impose or entail the self, making solipsism the tautological truth it is?
It is not only a belief that there is an outside to the self (which is the solipsistic fact), but also it is a belief that there is an outside counterposed. For why should it not be that there is only outside, and the self is illusion? I suggest a modification of solipsism: I can only know there is this; not that, positing else, this is in else or (even) in relation to else. And I cannot know there is else. Neither can I know that this is not what this would have me assume else is. I only experience this, and because I do not and cannot even know whether there is an else to it, I do not and cannot know it. In this I only can believe in this.