Empirical evidence, for one. And the existence of fine-tuning, which allows you to artificially influence how a model responds to questions. This means we can't just ask an LLM, "do you see red?" I can't really even ask you that. I just know that I see red, and that many other philosophers and scientists in the past seem to agree with my experience, and that it's a deep, deep discussion which only shallow spectators are currently drawing hard conclusions from.