The Functionalist Trilemma

Functionalism is among the most influential positions in the philosophy of mind. Its central idea is that mental states are determined not by their material constitution but by their functional or causal role. This gives rise to the possibility of multiple realization: if the same functional organization can be realized in biological and artificial systems, then, in principle, the same mental states should also be possible in different material substrates.

This paper argues that this inference does not hold for phenomenal consciousness. Functional equivalence initially establishes equivalence only with respect to functionally specified properties. For phenomenal equivalence to follow, an additional bridge premise is required according to which a particular functional organization is sufficient for experience. There are three argumentative possibilities for such a bridge. It may be stipulated definitionally, justified as an empirical generalization, or asserted as a fundamental psychophysical principle, for example as a necessary identity or a fundamental law. In the first case, consciousness is not explained but functionally redefined. In the second case, empirical evidence from biological systems is insufficient to establish substrate independence. In the third case, the transition from function to experience is carried by an additional principle rather than by the functional description itself.

This problem can be formalized as a factorization problem. A functional description is an abstraction that maps different realizations onto the same functional class. Functionalism about consciousness additionally claims that experience factors through precisely this abstraction, and therefore remains invariant within every functional equivalence class. This invariance does not follow from the abstraction itself. It is the central additional assumption of functionalism. Functionalism can therefore describe functional equivalence, but it cannot derive phenomenal equivalence from functional equivalence alone.