Knowledge in its full and valid sense is possible only of the immutable, the fixed} that alone answers the quest for cer- tainty. With regard to changing things, only surmise and opinion are possible, just as practically these are the source of peril. To ascientific man, in terms of what he does in in- quiry, the notion of anatural science which should turn its back upon the changes of things, upon events, is simply incom- prehensible. What he is interested in knowing, in understand- ing, are precisely the changes that go onj they set his problems, and problems are solved when changes are interconnected with one another. Constants and relative invariants figure, but they are relations between changes, not the constituents of ahigher realm of Being. With this modification with respect to the object comes one in the structure and content of "experience." Instead of there being afixed difference between it and some- thing higher rational thought there is adifference between two kinds of experience jone which is occupied with uncon- trolled change and one concerned with directed and regulated change. And this difference, while fundamentally important, does not mark afixed division.