Epiphenomenon

An epiphenomenon is a secondary phenomenon that accompanies or parallels a primary phenomenon. The term is used across multiple fields with two distinct senses: one implying a known causal connection, and another in which no causal relationship is assumed or asserted. These two uses inform different interpretations in philosophy, psychology, medicine, and scientific theory.

Epiphenomena in Metaphysics and Causation

In metaphysics, an epiphenomenon refers to any effect that arises from a cause but is not the principal effect under consideration. When a primary event produces two outcomes, only one of which is of central interest, the other is classed as an epiphenomenon.
Epiphenomena often challenge theories of causation. In simplified counterfactual analyses, such as those associated with David Lewis, causal claims are evaluated using conditionals of the form “if event A had not occurred, event B would not have occurred”. If A causes B, and B has an epiphenomenon C, then the counterfactual condition “if A had not occurred, C would not have occurred” is also true. According to the counterfactual view, this may incorrectly imply that A caused C in the primary sense, which contradicts ordinary causal judgements. Such scenarios illustrate that counterfactual dependence alone may be insufficient for identifying meaningful causal relationships.

Epiphenomena in Philosophy of Mind

In the philosophy of mind, an epiphenomenon is a mental event caused by physical processes but lacking the capacity to influence the physical world. This notion forms the basis of epiphenomenalism, which asserts that mental states arise from neural activity but do not themselves exert causal power.
Two principal forms of epiphenomenalism are discussed:

• Strong epiphenomenalism, in which mental phenomena are produced solely by physical processes, and cannot cause either physical or mental effects.
• Weak epiphenomenalism, in which mental phenomena may be influenced by both physical and mental causes, yet cannot produce any physical consequences.

Under these views, the physical world proceeds independently of mental causation, with mental events occurring as a dependent but non-interventionary parallel domain. Instrumentalist approaches modify this stance by permitting mental events to cause physical events when the mental description can be fully reduced to a physical summary, thereby preserving the primacy of physical causation.

Epiphenomena in Medical and Clinical Contexts

In medical terminology, the term is most often used without implying causation. Here an epiphenomenon describes a secondary or accompanying occurrence that may be related to, but is not necessarily caused by, a primary clinical condition. The usage indicates uncertainty regarding causality or acknowledges that only correlation is known.
Medical signs, symptoms, syndromes, and risk factors can all function as epiphenomena. For instance, an observed association between antibiotic use and increased breast cancer risk does not imply that the antibiotic is causative; instead the underlying inflammation associated with infection is the primary process. The antibiotic and the cancer risk thus represent parallel branches arising from the same underlying condition.
This distinction is often illustrated with a tree metaphor: the root represents the primary cause, the trunk an intermediate physiological response, and the branches secondary consequences. Epiphenomena are the branches—related by correlation, not necessarily by direct causation.

Epiphenomena in Electromagnetism

In electromagnetism, electrons are central to explaining electric currents, yet classical electromagnetic theory often treats these particles as secondary to the governing field equations. Under this perspective, electrons appear as epiphenomena of the electromagnetic field, whose behaviour is fundamentally described by Maxwell’s laws. Although not a universal view, this interpretation underscores the conceptual separation between field-based accounts of electrical phenomena and particle-based descriptions.

Epiphenomena in Propositional Theories of Cognition

Within cognitive science, the term is invoked by propositional theorists such as Zenon Pylyshyn, who argue that mental images are not foundational to thought but instead arise as by-products of underlying propositional, or meaning-based, representations. According to this account, people engage in reasoning and conceptualisation via abstract relational structures rather than visual imagery.
Mental images are therefore considered epiphenomena: secondary constructs that emerge when individuals voluntarily or involuntarily generate visual forms. Their variability according to prior expectations suggests that meaning precedes imagery. Since many concepts lack visual form, propositional theory maintains that imagery cannot be the basis of cognition. However, the classification of images as epiphenomenal remains difficult to verify, as subjective experiences resist empirical falsification.