Change, then, involves two aspects. It is a succession of events in time, and these events are connected by a systematic unity in such a way that they form the expression of a plan or law of structure. The series of successive states which make up the history of a thing are the expression of the thing’s nature or structure. To understand the thing’s structure is to possess the key to the succession of its states, to know on what principle each gives way to its successor. And similarly, to have complete insight into the nature or structure of Reality as a whole would be to understand the principles according to which every transitory event in the history of the Universe, regarded as a series of events in time, is followed by its own special successor. It is evident that, in proportion as our knowledge of any thing or system of things approaches this insight into the laws of its structure, the processes of change acquire a new character for us. They lose their appearance of paradox, and tend to become the self-evident expression of the identity which is their underlying principle. Change, once reduced to law and apprehended as the embodiment in succession of a principle we understand, is no longer change as an unintelligible mystery. We should bear this in mind when we reflect on the doctrine of Plato that the physical world must be unreal because the scene of incessant change. Such a view is only to be understood by remembering that before the invention of the mathematical methods which have enabled us with such conspicuous success to reduce physical phenomena to orderly sequence according to law, the physical world necessarily appeared to the philosopher a scene of _arbitrary_ change following no recognisable principle. Change, so far as understood in the light of its principle, has already ceased to be mere change.[102] § 3. _Ground and Consequence._ In the technical language of Logic, the underlying principle of any system is called its _Ground_, the detail in which the principle finds systematic expression is called its _Consequence_. Ground and Consequence are thus one and the same systematic whole, only considered from two different points of view. The Ground is the pervading common nature of the system, thought of as an identity pervading and determining the character of its detail; the Consequence is the same system, looked at from the point of view of the detail, as a plurality of differences pervaded and determined by an identical principle. The understanding of a process of change thus clearly consists in bringing it under the principle of Ground and Consequence. In so far as we are successful in detecting a principle in the apparently arbitrary succession of events, these events become for us a system with a common principle of structure for its Ground, and a plurality of successive states as its Consequence.