I have written in an other post that we 'believe' things which might not appear to be true. For example, if we assume every event has a cause, the idea of 'free will' would be absurd. Also since all our obsevations of the world are in the final analysis in our minds, it is impossible to "prove" there is a world outside our perceptions. However, almost all sane people believe there is a 'world' outside our minds, and that I have a choice to sit down or stand up. Free will and the existence of a 'world' outside our minds cannot be proven but we accept these ideas as almost certain. (See as suggestive, Newman's Illative Sense.)
Now the Catholic position on abortion, starting with the idea that human persons come into being at the time of conception is simply not believeable to the very great majority of perple. I believe it was Cardinal Hume who said when he was in a room with fertilized eggs in a petri dish he was there with each dish containing a person. Now assume a fire were to destroy this laboratory, and it had 15 fertilized eggs would we say 15 people were killed. Cardinal Hume would say "yes." The overwhelming majority of people would say "no" because it affronts their belief that what was in the dishes were "persons".
Most people do not accept that a tiny number of undifferentiated cells are "persons". Most people do not accept that human matter that has no brain waves, no feeling, no ability to experience pain etc. is the same as a human being.
And in the practical emotive sphere, a person who has a miscarriage at 6 weeks does not react as if they had lost a born child. (Although I did find one person in my blogging, who argued that a miscarriage was just as traumatic as the death of a one year old.) A women who exersizes while pregnant and loses a fetus is not charged with involuntary manslaughter.
Obviously endless examples could be given e,g. fetus' are not counted in the population. But the point is clear. No matter how many clerics fulminate that 1 or 2 cells are a person, people just don't buy it. Let me make it clear, these cells are POTENTIAL persons, but the modifier shows that they are not persons. The cell or cells are human life, but so are my fingernails, so are cancers and on and on.
In sum then, common sense makes a clear distinction human life and a person. My kidneys are human life; they are not a person. Sophistry about potentiality does not change our thinking. After all every acorn is a "potential" oak tree, but when I throw away an acorn no one says I threw away an oak tree. Jack