Seasoned pro-life activists often come across supporters of legal abortion who say that consciousness should be the defining criteria for "personhood".
The typical pro-life reply is that "consciousness" is an arbitrary line of demarcation, and that if it were implemented it would exclude certain categories of human beings. Then the pro-lifer goes on to show that ontologically the human being at conception is the same being at birth, and makes the argument that both are entitled to the same rights.
I suggest a new tactic on top of the old ones.
Demand that their opponents justify their choice.
First, demand a definition of consciousness.
Philosophers who study consciousness cannot themselves supply an adequate and all-encompassing definition of consciousness. All definitions are somewhat ambiguous.
For something that is the defining marker of legal and social personhood, that is very dangerous.
Second, ask how we would measure it.
Scientists themselves have a hard time making a measure, and medical personnel may not agree on the interpretation of the data.
And consider this:
Suppose a child is born with part of his brain missing.
How do we measure the child's consciousness is sufficient to grant him legal personhood? How do we make sure that no such child is discriminated against?
It is perfectly plausible that he might be unable to communicate the required level of consciousness, although he possesses it.
And how do we make sure that the criteria for consciousness is only applied to humans, and not apes or (in the future) robots? And how do we make sure that no other categories of human beings are discriminated against?
We have to show that our opponents' concept of humanity is a flawed one, and that the only intellectually sustainable model of the human being is one that makes his identity synonymous with his physical existence.
I think we have to take our pro-life arguments to the next level from a philosophical standpoint. We not only have to prove that the unborn are homo sapiens, and assume that people will make the ontological equivalence between the born and the unborn. We cannot take it for granted that people adhere to notions of social equality based on notions on ontological equality, because the truth is many don't and don't have a sound philosophical foundation whatsoever.
We have to show that other models of the human being are ambiguous, and that their ambiguity leaves large number of people vulnerable to having their personhood denied.
When it comes to issues of personhood, the criteria for recognizing it must be obvious and easily discernible. You can't base human rights on uncertain criteria.