. But
is what is known as a differential equation in the
’s. By this we mean that it contains in its expression the differences in the values of the
’s at neighbouring points. What we wish to discover, however, are the precise values of the
’s, and not their mere differences in value. For this purpose the differential equation must be manipulated in a certain way, or, as mathematicians would say, integrated. Only after the integration has been accomplished can solutions be obtained. Now all differential equations have a character in common in that they yield classes of solutions. In order to single out that particular solution which answers to the problem we may be considering, it will be necessary to impose certain conditions upon it. In the present case, the differential equations of the gravitational field are of the second order, which means that two separate conditions will have to be imposed.