The philosophical theism or the affirmation of God as First Cause of the being of entities was the main issue of Christian philosophy, along with the existential unity of the human person and the intellectual validity of the access to being – or realistic noetics. The development of the proofs of the existence of God was already a classic issue when thomistic systematisation collected a large part of the previous Christian philosophical heritage.
Following Plato’s thought, St. Augustine explicitely formulates the rational access to God through three ways: the proof of eternal truths, the proof of desire for happiness and the socalled deontological argument. In the proof of eternal truths, St. Augustine states that certain truths have necessity, immutability and eternity, features that they have regardless of contingent beings.
These necessary truths are previous to the existence of contingent beings and can not rely on them; therefore, they must rely on the existence of a necessary Substance. Detractors of this proof claim, justifiably, that it lacks demonstrative value, for necessity and eternity can not be placed in the same order. The argument reaches God in an ideal order, from eternal truths and by inner necessity, and doesn’t descend into its real noetic basis in being as such.
There is, then, an illegitimate step from the ideal towards the real. The augustinian proof of the existence of God by the desire for happiness is also known as the eudemonological argument: every natural desire implies the real existence of what is desired; since man has desire for God by natural need, God must exist. It is also necessary here to refuse the apodictic demonstration of the existence of God from the desire for happiness. On one hand, it is not true that every desire implies the existence of what is desired and, on the other, it is also incorrect that man naturally feels desire for God but, in any case, man feels desire for good in general.
Finally, the deontological argument tries to find out if one can correctly understand the existence of God as supreme Legislator from the knowledge of the natural moral law. The argument proceeds as it follows: in human nature there is a knowledge of a natural moral law. This law is necessarily caused, and proceeding towards infinite in the series of legislator causes is impossible; therefore, the existence of a first legislator cause must be accepted, which corresponds to the nominal definition of God.