I’d put forward the argument that they are fundamentally AI because of Nick and Curie. I mean, I know humans can be brainwashed, but synths are literally programmed.

When the pre-war Nick Valentine had his brain scanned by CIT and then the Institute created the metallic companion we meet in-game, I feel confident in saying that they didn’t bring the original Nick Valentine back from the dead, they coded an AI that translated a bunch of ones and zeros from that scan and used them to make a replica of a dead man’s mind.

On the other hand, when we move Curie, the undeniably inhuman Ms. Nanny programmed by Vault-Tec scientists, from her robot body to a synth one, I don’t mourn the death of the original body, because an AI can be transferred from one chassis to another, all you are doing is moving the code.

Fundamentally, I think synths are AIs that can mimic a human mind to include memories, consciousness, and sentience, but they aren’t human, since humans aren’t written in binary.