The Poison of Subjectivism

CS Lewis gives a good definition of subjectivism and points out the seriousness of this 'poison.'

To say that a thing is good is merely to express our feeling about it; and our feeling about it is the feeling we have been socially conditioned to have.
Out of this apparently innocent idea comes the disease that will certainly end our species (and, in my view, damn our souls) if it is not crushed; the fatal superstition that men can create values, that a community can choose its "ideology" as men choose their clothes.

Being subjective is a very common argument that people make. It essentially argues that your argument is invalid. Whats worse is that they use it to weigh how good something is. To this, Lewis writes:

If "good" and "better" are terms deriving their sole meaning from the ideology of each people, then of course ideologies themselves cannot be better or worse than one another. Unless the measuring rod is independent of the things measured, we can do no measuring.

Ultimately, Lewis has two ideas to get across:

1) The human mind has no more power of inventing an new value than of planting a new sun in the sky or a new primary color in the spectrum.
2) Every attempt to do so consists in arbitrarily selecting some one maxim of traditional morality, isolating it from the res, and erecting it into an unum necessarium.

Lewis makes the point that an ideology can make its advancements, but the ideology does not change and is essentially the same. He gives the example of changing from "Do not do to others what you would not like them do to you," to "Do as you would be done by." It is an advancement, but the principle stays unchanged.

Real moral advances, in fine, are made from within the existing moral tradition and in the spirit of that tradition and can be understood only in the light of that tradition. The outsider who had rejected the tradition cannot judge them. He has, as Aristotle said, no arche, no premises.

 I found this CS Lewis reading to be more philosophical than others. He relates back to the Moral Law, as we have read in Mere Christianity. I found it interesting that he explains how God neither obeys it or created the moral law. While I'm still confused at the point, I found this quote interesting.

The good is uncreated; it never could have been otherwise; it has in it no shadow of contingency; it lies, as Plato said, on the other side of existence.