> Human life is valuable because of critical properties
Specifically, the capacity for rationality and the capacity to choose among alternatives (insects feel things, too, in their myriad insect ways). And these properties, far from being properties among many, are definitive, constitutive, essential* to what it means to be human. (The instantiation of other human properties is always as human-specific instantiations rooted in these above essential properties; while a cat also feels something analogous to human anger and experiences something analogous to the human desire for food, they are not univocal.)
In other words: human value comes from the kind of thing humans are, which is to say rational animals.
And these essential properties exist in potentia during the embryonic stage. A rock does not have the potential to be rational, nor does a dung beetle at any stage, nor do even human gametes, as their development does not lead to a rational being. But at fertilization, from that first cell when a new human being comes into existence, we have a being in the most literal sense that has exactly that rationality scheduled, as it were. And the degree of rationality we express is always a continuum. How much rationality has developed in an infant? How much rationality does a toddler express? The teenager or even most adults? A bed-ridden person with Alzheimer's in old age? A comatose patient? To say that human life at some stage or other does not possess humanity is drawing lines in the sand, an arbitrary threshold that we choose to rationalize some action we wish to take that is opposed to the good of such a being.
Not actually, but it is not identical to nonexistence. It's capacity. If something didn't first have the potential for something, it could never become actual in that manner. And which potentials something has depends on the kind of thing it is. Thus, only human beings in their early stages have the potential to be adult human beings.
You're basically committing the error of Parmenides all over again.
No, it is literally identical to nonexistence. An acorn is not an oak tree. If I were looking for objects that satisfied the properties of having a trunk and providing shade, an acorn would not qualify.
On an arbitrarily large timescale, many things have the potential to become other things. Depending on your preferred theory of abiogenesis, some frothy chemical soup on early Earth had the potential to become, and did become, all of life. This does not give the soup moral value equivalent to all of life. What matters is what things are now, not the other things they could turn into.
Specifically, the capacity for rationality and the capacity to choose among alternatives (insects feel things, too, in their myriad insect ways). And these properties, far from being properties among many, are definitive, constitutive, essential* to what it means to be human. (The instantiation of other human properties is always as human-specific instantiations rooted in these above essential properties; while a cat also feels something analogous to human anger and experiences something analogous to the human desire for food, they are not univocal.)
In other words: human value comes from the kind of thing humans are, which is to say rational animals.
And these essential properties exist in potentia during the embryonic stage. A rock does not have the potential to be rational, nor does a dung beetle at any stage, nor do even human gametes, as their development does not lead to a rational being. But at fertilization, from that first cell when a new human being comes into existence, we have a being in the most literal sense that has exactly that rationality scheduled, as it were. And the degree of rationality we express is always a continuum. How much rationality has developed in an infant? How much rationality does a toddler express? The teenager or even most adults? A bed-ridden person with Alzheimer's in old age? A comatose patient? To say that human life at some stage or other does not possess humanity is drawing lines in the sand, an arbitrary threshold that we choose to rationalize some action we wish to take that is opposed to the good of such a being.