What Vladimir said. The actual variable in the AI's programming can't be magically linked directly to the number of iron atoms in the atmosphere; it's linked to the output of a sensor, or many sensors. There are always at least two possible failure modes- either the AI could suborn the sensor itself, or wirehead itself to believe the sensor has the correct value. These are not trivial failure modes; they're some of the largest hurdles that Eliezer sees as integral to the development of FAI.