Premise 3 defense: if external forces determine your beliefs, you lose justification for knowledge. You become a "bag of beliefs, none of which are up to the bag." The mad scientist thought experiment illustrates this.

If naturalistic determinism is true: something other than you determines what you think and how you evaluate. You can't determine what you OUGHT to believe if something else determines what you WILL believe. Even if physics/chemistry accidentally produces true beliefs in you, it's like guessing correctly on a multiple-choice test — you got the right answer by luck, not knowledge. Socrates: knowledge = justified true belief; mere true belief "runs away" without the tether of rational grounding. Mad scientist thought experiment: if a scientist controls all your thoughts via brain implants, you cannot rationally affirm any belief — whatever you say in response is the mad scientist, not you. Replace "mad scientist" with "physics and chemistry" — same problem. Some atheists (Rosenberg, Harris, Dennett) are driven to deny their own existence to maintain naturalism. "To deny their existence of God, they must deny their own existence."