Mark Johnson is, of course, most famous for his two works with the linguist George Lakoff,
Metaphors We Live By and
Philosophy in the Flesh. You may want to read those books first as they contain the elements of their philosophy but I will attempt to summarize.
They claim that investigations of the brain reveal that humans think metaphorically. We experience the world from a young age and create our conception of the world through conflation of experiences. These "conflations" are sets of metaphors that structure the way we see the world. Examples are:
Happy is up (e.g. I'm feeling up)
Categories are Containers (e.g. Are tomatoes in the Fruit or Vegetable Category)
Time is Motion (e.g. Time Flies)
etc. etc.
According to Lakoff and Johson we use these metaphors among others to think. Now some of these metaphors seem close enough to direct human experience as to be practically universal but most of our metaphors are culturally and historically determined.
That means that claims to Universal Moral Laws are not supportable. I do not think they would be post-modernist exactly, in that I understand post-modernism (a la Beaudrillard) to be joyously relativistic and I think that the embodied philosophy of L&J says that there are standards from which to judge.
THe whole notion of
Moral Imagination is that the whole process of our morality is necessarily imaginative. We must investigate the appropriateness of our moral principles for given situations. Quoting
MI:
...(T)he kind of imaginative judgement widely recognized as appropriate to the making, experiencing, and evaluating of artworks can serve as a model of moral judgement, insofar as it is pervasively imaginative in many of the same respects. pp214-5
So Aesthetics =/= Ethics but Ethics»Aesthetics