I suspect most people would agree that immortality (provided such a thing exists) begins from the moment that we come into being. So far as durability is concerned an immortal has no worldly peer; he or she will survive not only bodily death but the end of time itself. The light of immortal consciousness is never dimmed. Once created, it is both imperishable and ever-present.
Only, I don't buy a word of it, and here's why.
In 1979 I was involved in an automobile accident. I'd been a passenger in an old pickup truck that not only had no airbags, it had no seat belts as well. My head smashed mightily into the windshield leaving me fully unconscious for what might have been a minute.
While I survived death that night by the skin of my teeth, I only later realized that it had destroyed any rational hope that I might have for immortality. Immortal beings do not lose consciousness; not for an instant; not for a minute; not for an eternity.
The very concept of immortality stands crucially on the notion that once human life exists a fully functioning body is no longer required for the maintainance of our conscious being. If I were immortal the instant that my brain switched-off, some other means to support my consciousness ought to have immediately switched-on. If this happens to someone so unfortunate as to have been run over by a train or blown to bits by a bomb, it surely ought to have kicked in whilst I was recovering from a knock on the head.
In my case, nothing switched-on until my brain had "rebooted" and come back online. That minute of time had gone missing. It had gone missing no less than the semi-eternity before my birth had passed unnoticed.
It might be argued that some level of brain activity was present while I was temporarily unconscious. Granted. Despite this, my presence-of-self during that minute wasn't worth a damn. This, I attest from first-person experience; or in my case, the lack thereof.
Imagine test driving a car that's advertised as being so durable that it will run flawlessly, and without maintenance, for the next one-hundred years. What credence would you give those claims if the engine quit, and refused to start again for a full minute before you'd even driven the car off the dealer's lot?
Bear in mind, the usual claim is that our immortal consciousness will be sufficiently robust to survive the heat-death of the Universe. But all it took was a smack on the head to temporarily bring mine to a screeching halt. Moreover, I've fallen unconscious twice since my accident in 1979, for reasons far less dramatic than embarrassing.
When I Am Dead, My Dearest, by Christina Rossetti
The philosopher, George Santayana wrote
"The fact of having been born is a bad augury for immortality."
I agree. And yet I think it would be a far tougher case to make if having been born, our lives consisted of a seamless, unflappable stream of consciousness; which is precisely what an immortal ought to expect.
...like any star, I have nothing
to burn but the life I love
William Matthews, E Lucevan Le Stella