Refining the Debate between Multiple Realisation and Mental Constraint Theses

Cansu Irem Meric

University of Manchester

Like many concepts in philosophy, there isn’t a consensus about what cognition is supposed to refer to. Some theorists argue that an organism is cognitive only if it instantiates the right kinds of mechanisms (i.e. neural architectures or their equivalents). This view known as the mental constraint thesis (MCT). While others hold that what matters is not the mechanism but the function: any system that flexibly represents, learns, and responds to its environment counts as cognitive, regardless of substrate. This view known as multiple realizability thesis (MRT). Despite extensive debate, no consensus has been reached on whether cognition should be mechanism-first or function-first.
In this paper, I argue that plants provide a uniquely valuable test case for adjudicating between these positions. Plant behaviour, including signal integration, adaptive plasticity, and learning-like processes, has increasingly been taken to suggest some form of cognition. The case allows four logical possibilities to be tested: (A) plants are cognitive and possess the relevant mechanisms (which supports MCT); (B) plants are cognitive without those mechanisms (which supports MRT); (C) plants lack cognition despite having the relevant mechanisms, (which undermines MCT); or (D) plants lack both cognition and mechanisms (which either undermines MRT or leaves both accounts intact but gains us clearer boundaries). The case of plants may reveal that either our current frameworks are too narrow or that cognition admits of gradations rather than a strict yes/no classification. Thus, instead of merely supplying empirical evidence for one side or the other, plant cognition challenges us to refine the very criteria by which we determine what it is to be cognitive.
The paper doesn’t aim to establish whether plants are cognitive, nor to defend either MRT or MCT. Rather, it argues that treating plants as “boundary” case would provide a useful way of refining the debate between MRT and MCT and helps clarify what would be required to make progress toward resolving it.

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