At the risk of being too focused on the actual question: Dreams are a function of the brain. Most people have experienced dreams of a sexual nature that result in orgasm with no physical manipulation of the genital organs. So that proves that not only can people "get off" with just their thoughts but also that it is rather common. Aside from the dream aspect it is also fairly common for people to orgasm prior to penetration. I had a 19 year old virgin boyfriend a few years ago and he ejaculated before he penetrated me. It is true that I was holding him penis and guiding it into place so there was some limited physical contract but I think he was ready to ejaculate anyway. So that again shows that mental excitement can be enough to trip the switch. On the other hand, other than in dreams, I have never been able to get off just from thinking. I have come close a few times while watching certain themes in porn but I usually end up touching myself when I get excited like that so I don't really know if just watching would take me there or not. It sounds like I need to set up an experiment or two.