Even if everything I perceive is a projection from my mind, it's not a random projection. Even if my senses are not transparent, their fabrications are not random. They make sense, follow logical rules, and are consistent with the assumption of an external world that matches those perceptions, and they're thus deserving of being taken seriously.
So you're saying, that in reality external to the mind, there is such a thing as a thumb?
I did a search about property dualism and I couldn't see much written here on this site.