  A third version of the Problem of Interaction appeals to conservation laws. The leading idea is simple: Soul–body interaction would have to change the amount of energy in the physical universe. When souls act, new energy would appear in, say, the brain. When souls are acted upon, some quantity of energy in the brain would vanish. But either scenario would contravene established conservation laws, which permit only the conversion and redistribution of energy (or mass–energy) within the physical universe, not its addition or subtraction.