Actually, I think this is where the root cause of your disagreement is — the fact that his response to your words was predetermined, doesn’t mean that it follows *your* intentions. The two points aren’t inextricably linked together.
Simple thought experiment to illustrate this — you have a slowly dripping faucet. Under this faucet, you place bowl, upside down. Each drip ends up sliding off the bowl in a specific direction, some to the left, some to the right, some up, some down, seemingly randomly. The physical reasons for this are all deterministic — the water has no free will, and any deviation is obviously caused by some physical parameters, be it a slight disturbance to the droplet, or some slight imperfection of the bowl, or some slight breeze, or any combination of factors. Observably complex behavior (“choosing” to take a different path down the bowl) is obviously driven by simple, yet incredibly sensitive deterministic mechanisms.
So, say your response is some slight external influence on the drop of water. Because of the complexity and interaction between other slight external influences, you might not be able to get the next water droplet to go down the path you intend, even though your actions are going to have a predetermined effect.
tl;dr — predetermined != predictable
And here’s where I think we hit the teleological wall — for all intents and purposes, we have free will. Even if our choices are actually predetermined by a combination of neutrinos hitting the myelin sheath around a single neuron in our brain, causing us to hit McDonald’s rather than Burger King, it feels like *we* are making the decision, rather than simply reacting mechanically to a set of inputs and physical forces.
Given that, regardless of what the mechanical reality of the world is, we should behave as if we have free will. Now, those people who won’t behave that way can take refuge in the “well, my reaction was predetermined”. For the people who do behave as if they have free will, perhaps the only disadvantage is that they take personal responsibility for their choices (which, as you might suspect, isn’t really a disadvantage in my book).
