Sorry, the "if" was in reference to "if objective values exist" and Mackie argued that they did not. Anyway, I don't have too much time, so I'm going to post a quote here from a paper written on Mackie, and provide the link. Hope it helps a bit:
"Mackie criticizes the nature of supposed objective values with the argument from queerness, which involves metaphysics and epistemology. The metaphysical problem with objective values is how they could carry a motivational force and how they could supervene upon natural features. Philosophers have failed to describe how it can be possible that an item of knowledge about what one should or should not do can automatically motivate an agent to act accordingly upon being understood. Furthermore, there is no explanation of how they automatically exist as a result of certain actions or features. What is the link between an action or its consequence and the moral value that prescribes or prohibits it?
Even if the assumption that these objective values exist is granted, the immediate epistemological problem is how one could ever come to know these values. Mackie points to the failure of the intuitionists and the absurdity of divining truths through mere contemplation to reject the notion of a moral faculty or intuition. If there is no such faculty, then the remaining possibility seems to be reason, but reason cannot even find the link between actions and their associated values, so it does not seem to provide a reliable means to find these values."
http://www.arches.uga.edu/~pritchea/phil3200paper2.htm