The only consistent and logical path when defining when life exists can be termed in its “life directedness.” What properties must a being possess to make it its end result? This is not a theological argument but falls within the realm of ontology and being.
What constitutes existence? Should it be defined by man or the physical world? By the very nature of man’s temporality, any human explanation of “when life commences” is nothing more than a arbitrary and capricious journey into rational conjecture. There are no degrees to humanness any more than are different gradations to the concept of existence. One exists or one does not.
Was it not the Cartesian view that a certain organ in the human body was the conduit between the physical and metaphysical realms? Even Descartes finally saw the spurious argument in this.
As an amoral entity, I see no philosophical difference between the demise of a second trimester fetus and a two week old baby. On one side, there is an argument of convenience and seniority of the mother. On the other side, there is the hucksterism relating to the divine intention of a supreme being.
As my struggle for clarity falters, I am always buoyed by the quote from Salieri at the end of the movie Amadeus:
“I will speak for you, Father. I speak for all mediocrities in the world. I am their champion. I am their patron saint…Mediocrities everywhere… I absolve you… I absolve you… I absolve you… I absolve you… I absolve you all…”