How far is too far?

After reading the post on the main page about a device that can detect mental fluctuations, i started thinking about what determines how far certain things are taken? Yes, it has obvious benefits but i dont think it should be used frequently or ever. Its intruding on someones privacy and just seems un-ethical. knowing what people are thinking would only further press the issue on true freedom. Phone tapping and things like that are already being scrutenized and looked at as to whether or not theyre constitutional, i think that this "mind reading" device is in the same category and should not be permitted.