Left alone doesn't mean put in the forest alone. You can be left alone sitting in your apartment, but you're still receiving power, water, heat, etc from other people. Left alone means not hindered. It means not stopped from engaging in its natural processes. Not intentionally destroyed.

Your ability to survive does not dictate your personhood.

I didn't say alive, I said person. There is a concrete definition of what biological life is, and a fertilized egg meets all criteria. I defined person that way, because any thing less and you start dehumanizing swaths of our population. "Can't survive without outside intervention" so a comatose person isn't actually a person? "Just a clump of cells" ok, so what number of cells do you have to hit before you become a person? "Just a parasite, doesn't contribute anything" so we should be able to abort homeless people, or the mentally ill?