I’m hugely indebted to Michael Madary for his visionary and incisive commentary. The commentary covers three topics – the nature of perceptual content, the structure of experience, and some practical implications of the PP (Predictive Processing) framework. Each one deserves a full-length paper in reply, but I will restrict these brief comments to the first topic – the nature of perceptual content. Should the PP vision prove correct, Madary suggests, this would transform our understanding of the nature and role of perceptual content, with potential consequences for the larger project of naturalizing mental content. Driving such sweeping and radical reform is (Madary argues) the PP emphasis upon the active contribution of the organism to the generation of perceptual states. There is an active contribution, Madary (this collection, section 2) suggests, insofar as PP depicts perceptual states as “generated internally and spontaneously by the internal dynamics of the generative model” (p. 3).
Such a claim clearly requires careful handling. For even the most staunchly feedforward model of perception requires a substantial contribution from the organism. It is thus the nature, not the existence, of that contribution that must make the difference. Elaborating upon this, Madary notes that ongoing endogenous activity plays a leading role in the PP story. One might say: the organism’s generative model (more on which later) is already active, attempting to predict the incoming sensory flow. The flow of incoming information is thus rapidly flipped into a flow tracking “unexpected salient deviation”. Identical inputs may thus result in very different perceptual states as predictions alter and evolve. An important consequence, highlighted by Madary, is that different histories of interaction will thus result in different perceptual contents being computed for the very same inputs. Different species, different niches, differences of bodily form, and differences of proximal goals and of personal history are all thus apt (to varying degrees) to transform what is being predicted, and hence the contents properly delivered by the perceptual process.
Those contents are further transformed by a second feature of the PP account: the active selection of perceptual inputs. For at the most fundamental level, the PP story does not depict perception as a process of building a representation of the external world at all. Instead, it depicts perception as just one part of a cohesive strategy for keeping an organism within a kind of “window of viability”. To this end the active organism both predicts and selects the evolving sensory flow, moving its body and sensory organs so as to expose itself to the sensory stimulations that it predicts. In this way, some of our predictions act as self-fulfilling prophecies, enabling us to harvest the predicted sensory streams. These two features (endogenous activity and the self-selection of the sensory flow) place PP just about maximally distant from traditional, passive “feedforward hierarchy” stories. They are rather (as Mike Anderson once commented to me) the ultimate expression of the “active perception” program.
Here too, though, we should be careful to nuance our story correctly. For part of maintaining ourselves in a long-term window of viability may involve not just seeking out the sensory flows we predict, but the active elicitation of many that we don’t! PP may, in fact, mandate all manner of short-term explorations and self-destabilizations. But such delicacies (though critically important- see Clark (in press) chapters 8 and 9) may safely be left for another day. The present upshot (Madary this collection, section 2) is simply that PP, instead of depicting perception as a mechanism for revealing “what is where” in the external world, turns out to be a mechanism for engaging the external world in ways that say as much about the organism (and its own history) as they do about the world outside. To naturalize intentionality, then, “all” we need do is display the mechanisms by which such ongoing viability-preserving engagements are enabled, and make intelligible that such mechanisms can deliver the rich and varied grip upon the world that we humans enjoy. This, of course, is exactly what PP sets out to achieve.