However, there is a second problem about the interaction. Namely, the question of "how" the interaction takes place, where in dualism "the mind" is assumed to be non-physical and by definition outside of the realm of science. The "mechanism" which explains the connection between the mental and the physical would therefore be a philosophical proposition as compared to a scientific theory. For example, compare such a mechanism to a physical mechanism that "is" well understood. Take a very simple causal relation, such as when a cue ball strikes an eight ball and causes it to go into the pocket. What happens in this case is that the cue ball has a certain amount of momentum as its mass moves across the pool table with a certain velocity, and then that momentum is transferred to the eight ball, which then heads toward the pocket. Compare this to the situation in the brain, where one wants to say that a decision causes some neurons to fire and thus causes a body to move across the room. The intention to "cross the room now" is a mental event and, as such, it does not have physical properties such as force. If it has no force, then it would seem that it could not possibly cause any neuron to fire. However, with Dualism, an explanation is required of how something without any physical properties has physical "effects".