What is the moral argument for the existence of God in Mere Christianity by C.S. Lewis?

C.S. Lewis advances the moral argument for the existense
of God in the first part of his seminal book of Christian apologetics, Mere
Christianity, but not before defending the reality of the moral law itself.
That is, the moral law presupposes a Lawgiver. According to Lewis, the moral law
consists of the fundamental rules by which the universe and all who reside in it are
bound. Although much energy has been expended in debunking belief in an objective
concept of right and wrong, Lewis argues the moment the sceptic opines "It isn't fair"
he has defaulted to the authority of a superior moral law. Arguments about moral
conduct, namely, that one action is superior to another, take for granted that some kind
of higher law exists, one to which appeal can be made the world over. This truth having
been established, Lewis moves on to its origin, since, he avers, everything in the
universe had to have a beginning. Philosophers advance two theories for this: One is
materialist, the other religious. In the former, the principles governing the universe
arose by chance. In the latter, since morality throughout the history of the world
appears in the form of laws and codes, it stands to reason that it originated in the
conscious mind of a transcendent Lawgiver - God.