Here was just what I wanted--on the one hand the whole wide universe of existence; and on the other hand a brief formula with which to label its potted essence. How breathlessly one was led on, with only such breaches of continuity as separate paragraphs inevitably impose, right away from the primitive fire-mist to one of Bach's fugues or the critical doctrines of Mr. Ruskin, guided throughout by the magic of differentiation. What if the modes of existence, dealt with in successive sections, were somewhat startlingly diverse! Was not this itself a supreme example of the evolution of that diversity which the formula enables us to interpret? For if there were a passage from the homogeneous to the heterogeneous, the more heterogeneous the products--inorganic, organic, and superorganic, as I learnt to call them--the stronger the evidence for the law. Only by shutting one's eyes to the light that had been shed on the world by evolution could one fail to see how simple and yet how inevitable was the whole business. If then differentiation be the cardinal law of evolution--for the correlative concept of integration receives no emphasis in this early essay--does not the universality of the law imply a universal cause? Just as gravitation was assignable as a _cause_ of each of the groups of phenomena which Kepler formulated; so might some equally simple attribute of things be assignable as the cause of each of the groups of phenomena formulated in terms of differentiation. Now the only obvious respect in which all kinds of Progress are alike, is, that they are modes of change; and hence in some characteristic of changes in general, the desired solution must be found. Thus we are led up to the statement of the all-pervading principle which determines the all-pervading process of differentiation. It is this: _Every active force produces more than one change--every cause produces more than one effect._[3] In the first part of the Essay many and varied facts are adduced to show that every kind of progress is from the simple to the complex. The aim of the second part is to show why this is so: it is 'because each change is followed by many changes'. From the beginning, the decomposition of every expended force into several forces has been perpetually producing a higher complication, and thus Progress is not an accident but a beneficent necessity. In a brief third part we are bidden to remember that 'after all that has been said the ultimate mystery remains just as it was. The explanation of that which is explicable does but bring out into greater clearness the inexplicableness of that which remains behind.... The sincere man of science, content to follow wherever the evidence leads him becomes by each new enquiry more profoundly convinced that the Universe is an insoluble problem.... In all directions his investigations bring him face to face with the unknowable; and he ever more clearly perceives it to be the unknowable'.[4]