This necessary relation between the _à priori_ and _à posteriori_ elements of knowledge is not a mere subjective law of thought. It is both a law of thought and a law of things. Between the _à posteriori_ facts of the universe and the _à priori_ ideas of the reason there is an absolute nexus, a universal and necessary correlation; so that the cognition of the latter is possible only on the cognition of the former; and the objective existence of the realities, represented by the ideas of reason, is the condition, _sine qua non_, of the existence of the phenomena presented to sense. If, in one indivisible act of consciousness, we immediately perceive extended matter exterior to our percipient mind, then Extension exists objectively; and if Extension exists objectively, then Space, its _conditio sine qua non_, also exists objectively. And if a definite body reveals to us the _Space_ in which it is contained, if a succession of pulsations or movements exhibit the uniform _Time_ beneath, so do the changeful phenomena of the universe demand a living _Power_ behind, and the existing order and regular evolution of the universe presuppose _Thought_--prevision, and predetermination, by an intelligent mind. If, then, the universe is a created effect, it must furnish some indications of the character of its cause. If, as Plato taught, the world is a "created image" of the eternal archetypes which dwell in the uncreated Mind, and if the subjective ideas which dwell in the human reason, as the offspring of God, are "copies" of the ideas of the Infinite Reason--if the universe be "the autobiography of the Infinite Spirit which has also repeated itself in miniature within our finite spirit," then may we decipher its symbols, and read its lessons straight off. Then every approach towards a scientific comprehension and generalization of the facts of the universe must carry us upward towards the higher realities of reason. The more we can understand of Nature--of her comprehensive laws, of her archetypal forms, of her far-reaching plan spread through the almost infinite ages, and stretching through illimitable space--the more do we comprehend the divine Thought. The inductive generalization of science gradually _ascends_ towards the universal; the pure, essential, _à priori_ reason, with its universal and necessary ideas, _descends_ from above to meet it. The general conceptions of science are thus a kind of _ideœ umbratiles_--shadowy assimilations to those immutable ideas which dwell in essential reason, as possessed by the Supreme Intelligence, and which are participated in by rational man as the offspring and image of God.