(1) You do what you do — in the circumstances in which you find yourself—because of the way you then are.The thrust of this argument comes in the empirical observation that everything we do has to have had either some biological or environmental origination, or simple chance. Every choice we make is based on a prior choice, based on a prior choice, etc. - all confined within an interaction between our brain tissue and our environment.
(2) So if you’re going to be ultimately responsible for what you do, you’re going to have to be ultimately responsible for the way you are — at least in certain mental respects.
(3) But you can’t be ultimately responsible for the way you are in any respect at all.
(4) So you can’t be ultimately responsible for what you do.
I think the trouble we have with this is that we feel as though we have free will, or a choice in our own agency that is free from our past. When we look at a set of options, we see nothing pushing us towards one or another and assume that we are originating a choice. But the reality is that the great majority of our mental processes are unconscious. We simply have no way of seeing every tiny droplet of unconscious impulse that churns within our psyche.
I'm reminded of the idea that everyone believes himself to be "above average". Similarly, few of us believe we are horrible people - destined to do bad things; or great people - destined to do great things. Somewhat ironically, those most likely to think in these terms are the mentally ill. Yet most great and terrible things are not done by the mentally ill at all. They are done by people who were likely just living their lives and well, one thing lead to another. If we think back on the best or worst things we have done, they were largely unplanned.
From the heroic acts of kindness to the selfish acts of impulsive cruelty, most were likely done with little to no thought at all. When we wonder whether we have free will, we tend to ask ourselves whether or not we could do some or another task. Like, say, scratching our elbow. The obvious answer is, "Why yes, I am in complete control!" But rarely in life are we in such a zen-like state of perfect consciousness. In fact, such an existence would be overwhelmingly exhausting. Thank God for autonomous behavior (I get much of my best thinking done while doing something else!).
The biggest trouble people have in accepting determinism is the idea that they might no longer in control of their lives, or that personal responsibility is impossible. The simplest answer is: you never were, so what is different now? We still have all the dreams and desires and flaws that we have always had.
But doesn't life then become pointless, says the nihilist? Well, it depends on what you thought the point was to begin with. If you thought you were playing this really interesting game in which you were the supreme controller of your own universe, like a little God, maybe this is a bit of a let down. But relax, aren't glad that you weren't really in control anyway? Personally, I feel it's a considerable burden lifted. But just in case you're feeling a tad claustrophobic, try this experiment: do something without causing it.
OK, that's a little determinism joke. You can't. Whatever you do, or do not do, will have been determined already. So even if you say, "FUCK THIS SHIT!" and run screaming naked out into the street you will only be someone who has decided to run screaming naked out into the street. I guess it is kind of like the ultimate claustrophobic nightmare. But hey, might as well get used to it, right?
This is getting long, so I don't have time to go into what determinism means for morality, or society at large. Which is unfortunate, because I think that's when things get really interesting. Another day!