I have seen moral subjectivists imply the following argument at times:
- Either morality is based on God or morality is subjective.
- Morality is not based on God.
- Therefore, morality is subjective.
The argument is rarely put this clearly, but I suspect it’s a pattern of reasoning many moral subjectivists follow. They start out as Christians, accept that God doesn’t exist, and conclude that there is no morality, since the only concept of morality they are familiar with is supernatural. (This premise could also be seen as suggested by much of the moral debate in certain areas of the internet, which is mostly between divine command theorists and moral subjectivists.)
This is a bad argument because the first premise is unjustified – there is no reason why objective morality would have to be based on God. This is a Christian premise that there is no reason to accept on a secular worldview.
On a secular worldview, objective morality is much more likely to be based on the desire to live a good life. Everyone reading this likely wants to be happy, and you will be happier if you develop certain character traits in yourself over time. One example of such a character trait is productivity. People who are characteristically productive are likely to be happier than people who are lazy, because they will have accomplished more, so they will have higher self esteem, and they will likely be materially better off (more so than they would have been, at any rate).
This is the approach to morality moral subjectivists need to be considering when they make their case for moral subjectivism, not the much less plausible supernaturalist approach. We don’t use the pattern of reasoning “not supernatural, therefore subjective” in scientific issues, so we shouldn’t use it in philosophical issues.