Mackie offers two queerness arguments in Ethics (1977) against the reality of moral properties. One offers a metaphysical argument and an epistemological argument against moral realism. My concern here will be Mackie's epistemological argument.
The argument goes something like this: even if there were objective moral values, we would have no way of knowing about them. Since they are not natural properties or reducible to natural properties, ordinary perception does not let us perceive them. So we have no way of knowing anything about them without positing some "intuition" that seems too odd to be acceptable.
Mackie believes that perception is the basis and foundation for knowledge, and the only things we can perceive are natural properties and objects. Objective values, whatever they may be, are not perceptible objects or natural properties. Much as we have learned from Moore, whatever moral properties are, they are non-natural. And our only means of learning about them is through some kind of moral intuition.
If objective values cannot be perceived, are mysterious in nature, and cannot be known by us, then they are beyond the pale for Mackie. The imperceptible should be eliminated from our cognitive repertoire.
The force of Mackie's epistemological queerness argument depends on objective values having no influence upon us. For instance, we can think of perception as being the impression of objects upon our senses. Objects "influence" our senses in order to be perceived. If something is determined not to be capable of impressing itself upon us or influencing us, then they cannot be known. Moral properties, however, are not perceivable and, thus, have no influence upon us, according to Mackie, so they cannot be known.
Two questions we should have for Mackie is whether objective values have no influence upon us, and whether we should believe that for moral properties to be truly objective they should not have any influence upon us. If objective values were not able to influence us, then we could not know anything about things that were objective. It would not be possible to apprehend something objective. If we cannot apprehend something objective, then objectivity is beyond our comprehension. However, how could that be if we are truly talking about objective values, e.g.? Something that is not known to us or that could not influence us shouldn't be able to be talked about. They'd literally be beyond our apprehension. Yet, Mackie seems perfectly happy to talk about how we cannot know what is not perceptible.
One of the characteristics of objectivity is that it is objective. Mackie has already said that we cannot perceive things that are objective because they have no influence upon us. So, the result is that Mackie's argument, if it is indeed correct, works against not only objective values but also objectivity