Philosophy at its best is the study of basic reality. It helps people to think more accurately. At its worst, philosophy is irrelevant or malevolent. Let the buyer beware.
One way to divide philosophy is into ethics, epistemology and metaphysics. Each can be ruined by being relativized.
The classic expression of ethical relativism is Pope Francis’s infamous “Who am I to judge?” In moral relativism there is no morality for all, just for individuals. Ethics is subjective (relative to the subject who is ethicizing), not objective (the same for everyone.)
Epistemological relativism says “Your truth is not my truth.” Everyone (or maybe every tribe) has his own truth. Alethic relativism (it sounds better in Greek) makes all truth relative to the subject, the individual.
These last two have been popular for decades, because relativism means you cannot be wrong. But there’s a new kid on the relativistic block. When the postmodern dude or dudette says “I self-identify as a[n] insert descriptor here” he is spouting ontological relativism.
Ontology is a subset of metaphysics, the study of the foundational principles of reality. Ontology deals with existence itself: What does it mean for something to exist? What are the possible modes of existence? What things really do exist? Ontological relativism expresses the idea that the nature of your existence, your very substance, is up for grabs. What you really are is your own choice.
In philosophy, “what you really are” could be called your substance. “Substance relativism” is another name for ontological relativism.
The appeal of substance relativism is the same as the appeals of the other relativisms: Under moral relativism you cannot be bad. Under alethic relativism you cannot be mistaken. And under substance relativism, your soul is free to be whatever it wants.
Under relativism, you’re off the hook. You have a perpetual get-out-of-jail-free card. And you have a magic ring that can make you be whatever you want to be. If reality is displeasing, Presto! Just change reality.
That’s the necessary endpoint of demanding freedom without acknowledging God, the ultimate authority and determiner of reality. If man really is the measure of all things, then all things (no exceptions) are whatever he wants them to be.
The only way out of this postmodern nightmare is to acknowledge the true and living God, the God of the Bible.