By assuming a deterministic physical universe, the objection can be formulated more precisely. When a person decides to walk across a room, it is generally understood that the decision to do so, a mental event, immediately causes a group of neurons in that person's brain to fire, a physical event, which ultimately results in his walking across the room. The problem is that if there is something totally non-physical "causing" a bunch of neurons to fire, then there is no "physical" event which causes the firing. This means that some physical energy is required to be generated against the physical laws of the deterministic universe—this is by definition a miracle and there can be no scientific explanation of (repeatable experiment performed regarding) where the "physical" energy for the firing came from. Such interactions would violate the fundamental laws of physics. In particular, if some external source of energy is responsible for the interactions, then this would violate the law of the conservation of energy. Dualistic interactionism has therefore been criticized for violating a general heuristic principle of science: the causal closure of the physical world.