The diversity of phenomena which compose the universe is but apparent; the physical forces, however varied they may appear, are but diverse manifestations of the same principles, always active, but the effects of which often vary their mode of application as well as the duration of their action. In like manner, the most simple phenomenon requires the co-operation of a multitude of diverse actions regarded by us as so many distinct forces. Thus there is not an isolated fact in nature, nor one in disagreement with universal order. Every phenomenon is one of the consequences of universal laws. If these laws are not clearly known in the complexity of their applications, the facts which are ever before us indicate at least their character and the direction which they take. Thus critical minds, enlarged by the school of scientific experience, refuse to accept speculative systems and supernatural theories which involve the existence of phenomena in the universe in disagreement with the principles which govern it. A fact of this sort is proclaimed, science subjects it to scrutiny, and generally recognises that it is but the effect of forces analogous to those which occur in phenomena already observed, but acting in a different manner.