Here’s a quick statement of my current perception of the relationship between science and ethics. What it boils down to is this: in theory, science can determine how we ought to behave, but a) it would have to be magic future science, and b) you have to accept that there is no external ‘ought’. Let’s just do it in dot-point form:
1) There is no external ‘ought’. No rational rule or natural law or God is handing down a code of ethics. Our only motivations to do good (or way of figuring out what is good in the first place) are our own sentiments of kindness and compassion and so on.
2) Sentiments of kindness and compassion correspond to certain brain-states which are, in theory, measurable by magic future brain scans. Indeed, one day brain scans may be the best method of measuring the nature of our sentiments.
3) Therefore science, through the medium of brain scans, may one day be able to quantify our inherent ‘good’ sentiments to the extent that it can tell us what course of action will best satisfy our positive impulses.
Now I think this argument works pretty well. If you disagree with (1) you’re in good company – Kant, Plato, Aquinas, and so on – but as I’ve already detailed I side with Eliot and Foot. It’s hard to disagree with (2), but I think you might do it on the grounds that ‘sentiments’ are culturally constructed and so any brain scan is going to have unavoidable cultural bias. If you accept (1) and (2), (3) follows automatically.
The problem I have with the kinds of people putting this argument forward – Richard Carrier, Sam Harris – is that they’re not making it clear that the kind of ethics science can determine is not what most people mean by ethics. Accepting Carrier and Harris’ argument means accepting a pretty radical philosophical take on ethics: that there are no rules or principles, and that it is really all down to how we feel. Which is correct, I think – but that’s what they need to argue, and in general they just pass it over as if it were obvious.