Introduction:
By “epiphenomenonal” I do not mean those perfectly valid descriptions appropriate to the context of physics and biology to articulate those phenomena which can be termed non-primary insofar as their effects are correlated with some relevant primary effects, but are not suspected to be their cause (see: Epiphenomenon subsections “Medicine” and “Electromagnetism”). Instead I mean the usage common to materialist theories of mind, i.e. the doctrine that consciousness exists, but is fundamentally acausal in the physical sense (as though there could exist some rupture between physicality and causality). In this sense consciousness does not affect the brain in any meaningful way, but is “epiphenomenal” - an illusory and metaphysically impotent byproduct of our third-person ontology.
Argument:
“Epiphenomenon” in this sense is an anti-concept, and more specifically, a stolen concept. When speaking to the referents of the concept of “nothing” in the appendix to Introduction to Objectivist Epistemology, Ayn Rand describes such negative concepts as “purely relative”. She remarks later on that “[n]on-existence - apart from what it is that doesn’t exist - is an impossible concept. It’s a hole - a literal blank, a zero”. In our case of the concept “epiphenomenon”, the relative distinction has been collapsed - the referent in question is both “non existent” and the “what it is”. The omissions relevant to the formation of the concept “nothing” are the totality of the measurements belonging to the existents whose absence is being signified. The omissions relevant to the the formation of the concept “being” are the the totality of the measurements of the measurements belonging to the existents whose existence is being signified. In collapsing the just-mentioned distinction, the measurements and the measurement’s measurements become one, absolving the relative character needed to produce anything of sense about an absence of being. This “sense” derives from the existent (read: causal) nature of all productions of knowledge and principles known. Put very simply, the attribution of acausality contradicts the requirements of knowing an existent to attribute. The absoluteness of reality and the principle of no metaphysical hierarchies guarantees the nonexistence of any gradations of existence, including the gradations of existence relative to putatively known existents.
Conclusion:
The adjectival form of “epiphenomenon” common to those materialist fetishizations of the human mind’s nonexistence is an anti-concept, and just another poor way (albeit a fashionable one) of attempting to side-step the axiom of consciousness.