I think by his conclusion that he must exist since he is having doubts gives more foundation for a soul, but not yet any reason that he still can't just be a brain in a vat.He doesn't seem to consider anything but a soul, when I suppose it could just be electric stimulation of the physical brain in the jar. He puts a bigger concept to it with the word "soul".This has a deeper meaning to it than just mental stimuli, for him back then, and still for us now.SO does the fact that he even has a soul concept show that he is more than just an illusion,or plaything for the Arch Deceiver?