Because Hume was historically ignorant of Mirror Neurons, Hume speculated the following;Hume identifies “sympathy” as the capacity that makes moral evaluation possible by allowing us to take an interest in the public good (T 3.3.1.9).
Because of the central role that sympathy plays in Hume’s moral theory, his account of sympathy deserves further attention. Hume tells us that sympathy is the human capacity to “receive” the feelings and beliefs of other people (T 2.1.11.2). That is, it is the process by which we experience what others are feeling and thinking.
https://iep.utm.edu/humemora/
But now we have the physical referent, the Mirror Neurons as a matter of fact that represent the mechanics of sympathy [Empathy].That is, it is the process by which we experience what others are feeling and thinking. This process begins by forming an idea of what another person is experiencing. This idea might be formed through observing the effects of another’s feeling (T 2.1.11.3). For instance, from my observation that another person is smiling, and my prior knowledge that smiling is associated with happiness, I form an idea of the other’s happiness.
My idea of another’s emotion can also be formed prior to the other person feeling the emotion. This occurs through observing the usual causes of that emotion.
Hume provides the example of someone who observes surgical instruments being prepared for a painful operation. He notes that this person would feel terrified for the person about to suffer through the operation even though the operation had not yet begun (T 3.3.1.7).
This is because the observer already established a prior mental association between surgical instruments and pain.
https://iep.utm.edu/humemora/
Since for Hume [if he was not ignorant of factual mirror neurons],
1. morality is related to sympathy [Empathy]
2. Empathy is justified by the fact of mirror neurons,
3. therefore there are moral facts as a matter of fact [the factual mirror neurons].
This factual moral fact of mirror neurons and the moral potential are not moral evaluations and moral opinions but real physical matter of fact that is related to empathy, thus morality.