There is a contingent of ethical thinkers who believe that our moral instincts are the best guide to deciding what’s moral. Our moral instincts, these folks argue, constitute a reliable epistemology for ethics in a similar way that the scientific method constitutes a reliable epistemology for physics (and all that silly, unimportant stuff that relies on physics, like biology and chemistry*).
But the evidence seems to indicate that our moral instincts arrived at their current state through evolution, just like our other instincts. And since evolution is imperfect, inefficient, and undirected, we are not justified in concluding that our moral instincts are perfectly reliable.
In fact, there’s a way of testing this. It arises from the observation that some of our moral instincts are broader than others. For instance, our penchant for fairness is broader than our aversion to lying. Lying is quite a specific area of moral interest, while fairness features in almost every moral topic.
The test of our moral instincts is therefore this: Are our more specific instincts consistent with the broader instincts they fall under? For instance, does our aversion to lying ever interfere with our desire to be fair? (Put differently, are there instances in which telling a lie would be the most fair thing to do?).
If a specific moral instinct prompts us to go against a broader moral instinct, then our instincts are not logically consistent with one another, and we need to rethink our reliance on those instincts. And the only way to determine if one instinct conflicts in a logical sense with another, is to take the broadest (and therefore most fundamental) of all the instincts, extend its logical structure down to the more detailed levels, and see what these more specific instincts ought to be telling us under conditions of logical coherence.
This, in essence, was the purpose behind my essay (now short book) on morality: To take a simple and fundamental idea like the desire for happiness, and see where it logically leads.
So, while moral instincts can certainly be useful as a quick guide to moral thinking, they should constantly be checked to see if they support the basic principles of the moral system being used.
* Only kidding.