I don't remember how it started, but we ended up talking about free will.
The argument basically ran like this:
If there was a demon that could know everything about you up until the instant before you make a decision it would be able to predict with 100% accuracy what you will choose. Most people have a definition of free will that runs somewhere along the lines of "Free will is being able to trick the demon." However, it is impossible to do any such thing. The reason for this is that our choices are based on our personality, which is based on our history. This personal history is made up of 3 things: genes (this may not be true, but it doesn't really matter for this argument), memories, and external stimuli. Memories are simply the storage of external stimuli, so they don't really matter either. But these forces that are acting upon you (external stimuli: friends, family, situations) to form your personality are not uncontrollable, but the way they act upon you is.
At this point my father argued that you do in fact control how your personality is formed. He seemed to think that you can choose which past experiences you remember, or at least how you remembered them. However, even if this is true, "you" are making that choice based on your personality, which is once again, based on your past experiences, therefore, not under control by you. (Even if "you" somehow had control of which memories you store and how you recall them, "you" are still nothing more than a collection of previous memories, and the previous 'you's are simply collections of previous memories. That causal chain can go all the way back up until your birth, which "you" could not possibly have chosen. Ergo, none of the choices you make in your lifetime are truly yours.)
This would usually be an effective disproof of the natural concept of free will, but my dad was not impressed. I went over it a few times and pinpointed where he was getting lost. It was around here:
If there were two identical versions of you in two identical universes about to make two identical choices they would always do the same thing. That was what he didn't agree with. So we simplified it. Say there's a neuron that, when it fires, will cause you to blink. If there were two identical universes with two identical firing neurons in two identical situations you would always blink in both universes. He agreed with that too. So I changed it so that the same neuron no longer controls your eyelid, but whether you'll choose toast or a banana for breakfast. He no longer agreed. Somehow he thought (and still thinks I would assume) that there's something that allows you to choose the outcome of your physical choices. He said that the whole is more than the sum of its parts. I asked him how that could possibly happen, and he had no idea, he just knew that it does.
So basically my dad believes that even though all your choices are based on standard physical processes that occur in your brain, and in any other situation the outcome of these processes would be completely predictable, your conscious decisions are somehow affected by an unknowable, metaphysical force. So, a soul. He denies this allegation, but that's really the only thing it can come down to.
How about the one person who made it through this whole post? What do you think?