It would seem therefore that there are three admissible laws of gravitation~\Eq{(62.2)}. Each can give precisely the same gravitational field of the sun, and all astronomical phenomena are the same whichever law is used. Small differences may appear in the cross-terms due to two or more attracting bodies; but as was shown in our discussion of the lunar theory these are too small to be detected by astronomical observation. Each law gives precisely the same mechanical phenomena, since the conservation of energy and momentum is satisfied. When we ask which of the three is the law of the actual world, I am not sure that the question has any meaning. The subject is very mystifying, and the following suggestions are put forward very tentatively. The energy-tensor has been regarded as giving the definition of matter, since it comprises the properties by which matter is described in physics. Our three energy-tensors give us three alternative material worlds; and the question is which of the three are we looking at when we contemplate the world around us; but if these three material worlds are each doing the same thing (within the limits of observational accuracy) it seems impossible to decide whether we are observing one or other or all three. \PageSep{144} To put it another way, an observation involves the relation of the~$T_{\mu}^{\nu}$ of our bodies to the~$T_{\mu}^{\nu}$ of external objects, or alternatively of the respective ${T'}_{\mu}^{\nu}$ or~${T''}_{\mu}^{\nu}$. If these are the same relation it seems meaningless to ask which of the three bodies and corresponding worlds the relation is between. After all it is the relation which is the reality. In accepting $T_{\mu}^{\nu}$ as the energy-tensor we are simply choosing the simplest of three possible modes of representing the observation. One cannot but suspect that there is some identical relation between the Hamiltonian derivatives of the three fundamental invariants. If this relation were discovered it would perhaps clear up a rather mysterious subject. \Section{63.}{Gravitational flux from a particle} \index{Flux!gravitational}% \index{Gravitational flux}%