Furthermore, a rational solipist does not avoid any of the questions faced by non-solipsists: wherever the latter asks "why is it that... and not something else?", the rational solipsist has the question "why does it seem that... and not something else?" Saying "it was designed that way" is no more of an explanation than "it is that way", and the rational solipsist will realize that this is so.
Perhaps you are thinking of brains-in-vats or simulations in their own simulated universe? If such an entity supposed that it was a brain-in-a-vat or a simulation, however, it would not be a rational solipsist, as these hypotheses presume a reality beyond their experiences (in any such case, of course, the non-solipsist brain-in-a-vat or simulation would be mistaken about what reality lies beyond its experiences, but nevertheless, there would be such a reality.)
A brain in a vat hallucinating a universe, is the same thing as just hallucinating a universe with no further reality, is the same thing as being in a particularly vivid dream, is the same a being a simulation.
The idea that a brain must exist somewhere in a vat to present this reality to you, came from your mind. If some of the rules of your reality are entirely disconnected with your senses, why not all?
In this regard, the inability to disprove solipsism is not sufficient to be a justification for believing it; if it were, the mirror argument would equally justify not-solipsism, and so a rational person should regard both arguments as inconclusive.
I only mentioned brains-in-vats and simulations because I wondered whether mypalmike had them in mind, and only to say that I don't think it matters whether or not the putative solipsist is either of these things.
Rather than continue in this vein, I will point you to an article which concludes with the section "The Incoherence of Solipsism":
One might even say, solipsism is necessarily foundationless, for to make an appeal to logical rules or empirical evidence the solipsist would implicitly have to affirm the very thing that he purportedly refuses to believe: the reality of intersubjectively valid criteria and a public, extra-mental world. There is a temptation to say that solipsism is a false philosophical theory, but this is not quite strong or accurate enough. As a theory, it is incoherent. What makes it incoherent, above all else, is that the solipsist requires a language (that is, a sign-system) to think or to affirm his solipsistic thoughts at all.
Designed for you by whom? And when? That is the problem.
