In Brett’s monthly letter, he points out the similarity between our ability to perceive (and have knowledge of) moral reality and our ability to perceive physical reality:
[T]he idea of self-evident moral truths represents the ground floor of moral reasoning. Such truths are known to us by way of our moral intuitions. An intuition is a faculty of knowing. It’s a reflective way of seeing something. It’s a way of knowing that is immediate and direct. You see something, you reflect for a moment, and you simply come to see the truth of the matter....
To help students grasp this concept better, I compared their moral perception to their sense perception. Sight via the eyes is the faculty we use to see the physical reality around us. If a dog walks in front of me, I see it and know it exists. If you asked me how I know, I would respond, “Because I see it. It’s self-evident. I just see that dog walking in front of me.” If you asked me to prove it, I would simply point to the dog and say, “Look! It’s right there. Can’t you see it?” If you said, “No, I don’t see any dog,” I would conclude there is something wrong with your sense perception, that your eyesight is not working properly. However, I would not conclude that I was not actually seeing a dog.
In the same way, our moral intuitions are a faculty we use to “see” the moral reality around us. I see that torturing children for fun is wrong, or murder is wrong, or dishonesty is a vice, and thus apprehend moral truth. If you asked me how I know, I would respond, “Because I see it. It’s self-evident. I just see that torturing babies for fun is morally wrong.” If you asked me to prove it, I would simply point to the action of torturing young children and say, “Look! The truth is right there. Can’t you see it?” If you said, “No, I don’t think torturing babies for fun is wrong,” I would conclude there is something wrong with your moral perception. However, I would not conclude that it was not actually wrong to torture babies for the pure pleasure of it.