Postmodernism is a philosophy. It is the philosophy that modern leftism is grounded in.
From Wikipedia...
Postmodern philosophy is a philosophical direction which is skeptical of certain foundational assumptions of Western philosophy, of the 18th-century Enlightenment, and of philosophy in general.[1] It questions the importance of power relationships, personalization, and discourse in the “construction” of truth and world views. Postmodernists deny that an objective reality exists, and deny that there are objective moral values. One’s idea of good and another’s idea of evil are to be equally valid from the viewpoint of a postmodernist.[2][1]
Postmodernists deny that an objective reality exists, and deny that there are objective moral values.
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Relativism by another name.