In conclusion, while in one sense the Word of God goes before us in the Scripture, in another sense one has to say that reason and evidence take the lead to the point where the partnership of reason and faith express the reality of a good marriage: each contributing their necessarily complementary and fertile difference.
Thus faith does not diminish but brings to perfection the natural good of reason; and reason does not diminish the content of faith but makes that content more apparent.
Secondly, it would seem that there has not only been an implicit development of the doctrine of ensoulment from conception, but there has also been a careful explication, particularly in Evangelium Vitae, of the process through which we come to know that, from the point of view of the natural law, we can know with rational certainty that it is wrong to kill the innocent human being that conception by definition begins.
In other words, it is not necessary that we be able to answer every question concerning something such as the beginning of life; rather, it is necessary to recognise just what we need to know and how our grace assisted use of the natural law74 will lead us as far as it is necessary to go in order to know how to act in conformity with the love of God for us.
Furthermore, and while this takes us into a philosophical discussion that is not possible to pursue here, it can be said that the making of a man effects a substantial change in the nature of the human sexual gametes: a substantial change which by definition entails an act of God.
Philosophically, therefore, there is reason to know that God acted in the beginning of each one of us just as radically as He acted at the beginning of creation.
This idea is biblically confirmed in many ways but particularly by Eve, when she says: "I have gotten a man with the help of the Lord" (Gn 4: 1).
Thus recognizing that God and the parents are not in the same sense authors of life, but as it were as a First and instrumental cause75, is itself, even considered philosophically, an indication that the use of reason can assist us to know the direction through which we will come to know what is true, right and good to do.
The whole tradition of the doctrine of substantial change is another aspect of the relationship between developments in Catholic theology and philosophy and the Catholic Teaching on the taking of innocent human life; however, it has not been possible to do more than simply indicate that this is the case.
Finally, one begins to see how appropriate it is that the moment of conception should be the moment of ensoulment.
For this would indicate that God has chosen an outward moment to manifest the creation of the soul.
In other words, God's creation of the soul is a truly hidden act - but this 'invisible act' has been given a suitably exterior and visible manifestation, namely the moment of fertilization.
Thus we can speak of the parental giving of the outward bodily sign as inherently ordered to the Creator's inward gift of the soul, and which together constitute the co-creation of the human person.