But before passing to these, we may observe that all the _à priori_ theorists, professing to conduct us to the desired conclusion on the level road of demonstration (while they all contradict their own principles, and furtively introduce the contingent facts of experience), have but a faint conception of the magnitude of the question at issue. To work out a demonstration as with algebraic formulæ, to contemplate the problem as one of mathematical science, under the light and guidance of the reason alone, and unaided by the moral intuitions, betokens a lack of insight into the very problem in question. The object of which we are in search is not a blank colourless abstraction, or necessary entity. Suppose that a supreme existence were demonstrable, that bare entity is not the God of theism, the infinite Intelligence and Personality, of whose existence the human spirit desires some assurance, if it can be had. And a formal demonstration of a primitive source of existence (_more geometrico_) is of no theological value. It is an absolute zero, inaccessible alike to the reason and to the heart, before which the human spirit freezes; and as a mere _ultimatum_ its existence is conceded by every philosophic school. The germs of the cosmological argument (as of the ontological) are found in the scholastic philosophy, though its elaboration was left to the first and second periods of the modern era. Diodorus of Tarsus, John Damascenus, Hugo of St. Victor, and Peter of Poitiers, have each contributed to the development of this mode of proof. It is the argument _à contingentia mundi_, or _ex rerum mutabilitate_; and may be briefly stated thus: If the contingent exists, the necessary also exists. I myself, the world, the objects of sense, are contingent existences, and there must be a cause of these, which cause must be also an effect. Go back, therefore, to the cause of that cause, and to its cause again, and you must at length pause in the regress; and by rising to a First Clause, you escape from the contingent and reach the necessary. From the observation of the manifold sequences of nature you rise to the causal fountain-head, as you cannot travel backwards for ever along an infinite line of dependent sequences.