Now, in a theory which considers this earth as placed in a system of things where ends are at least attained, if not contrived in wisdom, final causes must appear to be an object of consideration, as well as those which are efficient.  A living world is evidently an object in the design of things, by whatever Being those things had been designed, and however either wisdom or folly may appear in that design.  Therefore the explanation, which is given of the different phenomena of the earth, must be consistent with the actual constitution of this earth as a living world, that is, a world maintaining a system of living animals and plants.

Not only are no powers to be employed that are not natural to the globe, no action to be admitted of except those of which we know the principle, and no extraordinary events to be alledged in order to explain a common appearance, the powers of nature are not to be employed in order to destroy the very object of those powers; we are not to make nature act in violation to that order which we actually observe, and in subversion of that end which is to be perceived in the system of created things.  In whatever manner, therefore, we are to employ the great agents, fire and water, for producing those things which appear, it ought to be in such a way as is consistent with the propagation of plants and life of animals upon the surface of the earth.  Chaos and confusion are not to be introduced into the order of nature, because certain things appear to our partial views as being in some disorder.  Nor are we to proceed in feigning causes, when those seem insufficient which occur in our experience.

Animal life being thus considered as an object in the view of nature, we are to consider this earth as being the means appointed for that end; and then the question is suggested, How far wisdom may appear in the constitution of this earth, as being means properly adapted to the system of animal life, which is evidently the end.  This is taking for granted, that there is a known system of the earth which is to be tried—­how far properly adapted to the end intended in nature.  But, it is this very system of the earth which is here the subject of investigation; and, it is in order to discover the true system that we are to examine, by means of final causes, every theory which pretends to show the nature of that system, or to assign efficient causes to physical events.

Here then we have a rule to try the propriety of every operation which should be acknowledged as in the system of nature, or as belonging to the theory of this earth.  It is not necessary that we should see the propriety of every natural operation; our natural ignorance precludes us from any title to form a judgment in things of which we are not properly informed; but, no suppositions of events, or explanations of natural appearances, are to be admitted into our Theory, if the propriety of those alledged operations is not made to appear.  We are now to make an application.