So long as we can deal with simple types of relatedness, such as that which we call gravitative, in any given system of things regarded as isolated, we can express in formulae not only the rate of change within the system, but also the rate at which the rate of change itself changes. And these formulae are found to be generally applicable where like things are in a like field of relatedness. So that Spencer's persistence of force (at least in one of its many meanings) is replaced in such cases by sameness of differential equations. And in such cases we have no need for the word cause. Of course the value of the constants in any such formula depends upon the nature of the field of relatedness and of the things therein; and only certain systems, in which the relations are simple, or are susceptible of simplification, can be dealt with, at present, in this manner. It is imperative to remember that not only the rate of change but the kind of change differs in different relational fields--a fact of which Spencer took too little cognizance, so bent was he on some sort of unification at all hazards. Revert now to a field of gravitative relatedness, in which the motion of things is the kind of change, while the rate of change is expressible in a formula; may we not say that the co-presence of things in this relationship does imply certain motions and changes of motion within the system to which the term gravitative applies? There seems little room for ambiguity if we call what is thus implied the effect, and if we term those modes of relatedness which carry this kind of implication, effective. It may, however, be said that it sounds somewhat strange to speak of relations as effective. How can mere relatedness as such _do_ anything? What is implied by the effect is surely, it will be urged, a cause in the full and rich sense of the word--a cause which produces the effect. For what is here suggested is nothing more than a generalized statement of the truth that the relational constitution of the system being what it is, the changes are what they are! And so we come back to the conception of an agency which in some way produces the observable change--of a power which is active behind the phenomenal scene--of force and cause in the Spencerian sense. But, so far as scientific interpretation is concerned, this reference to Source--for such it really is--is useless. The gravitative system can be dealt with scientifically just as well without it as with it.