Others have pointed out the spots in the book where it states the more rules oriented things, so I'll get more historical. In Shadowrun, back in around 2nd to 3rd edition (I'm not sure the exact point) the first version of what would later evolve into the technomancers of today, the Otaku, or cybershamans as many where called (since that seemed to be the dominant type) appeared. One thing that was explicitly said was that the ability to walk to astral planes of magic, and the ability to walk in the world of the electron, where mutually exclusive. If you had one, you could never, under any circumstance, have the other. It was hinted (though not outright said that I remember) in both game and story books that both types of powers use many of the same parts of the brain, and were hardwired to run only as such.