Carried to its logical end, you can't fully rule out being the only mind that exists
Solipsism is the position that the only thing you can be absolutely certain exists is your own mind; other people and the external world might, in principle, be illusions generated by your own consciousness. René Descartes pushed skepticism to this edge in 1641, doubting everything that wasn't direct experience before trying to rebuild certainty from his own thinking alone. No philosopher seriously believes it, but nobody has found a fully airtight logical step back out of it either.