THE “NO EXISTING ALTERNATIVES” OBJECTION

5 THE “NO EXISTING ALTERNATIVES” OBJECTION

Near the close of her 1989 paper, Hannan remarks,

even if all conceptual schemes, including the conceptual scheme embodying the notion of rationality, are vulnerable to revision and overthrow, we have no possible way to reject rationality and prepositional attitude concepts until replacement concepts are suggested. And at this point, no replacement concepts have been suggested…. in the absence of plausible replacements for these concepts, or even the hint that such replacements might be on the horizon, don’t we have ample reason to bet against the eliminativist?

I doubt Hannan is misled on the point, but it is worth emphasizing that no one is suggesting that we move out of our current house before we have constructed a new one that invites us to move in. What EM urges is only the poverty of our current home, the pressing need to explore the construction of one or more new ones, and the probability that we will eventually move in to one of them.

On two other points, however, I believe Hannan is importantly misled. The latter quotation embodies an argument of the form, “If FP is currently the only boat afloat, isn’t this ample reason to expect that it will continue to be the only boat afloat?” The response is straightforward. No, it isn’t ample reason to expect that. On the other hand, it is ample reason for immediately gathering as much driftwood as we can, and for beginning the construction of alternative boats, if only to foster illuminating comparisons with our current vehicle, which after all is leaking at every seam.

The former quotation embodies a far more important misconception. Here in 1993, we do have some very specific and highly promising “replacement” concepts under active exploration. They are now the prime focus of several new journals and they have been under vigorous exploration for over a decade at several centers of cognitive and neuroscientific research. These are the ideas mentioned briefly in (3) above. I can give only the flavor of this new approach here, but that much is quickly done.

One of the basic ideas of this new approach has some instances already familiar to you. Consider the momentary picture on your TV screen. That representation of some distant scene is a pattern of brightness levels across a

184 POSTSCRIPT coherent sequence of such patterns represents the behavior of that distant portion

of the world over time.

A very similar case, this time in you, is the momentary pattern of activation levels across the 100 million light-sensitive cells of your retina. The temporal sequence of such patterns represents the unfolding external world. A further example is the activation pattern, and the sequences thereof, across the millions of auditory cells in the cochlea of your inner ear. Here, of course, the “semantics” of the representation is not “pictorial” as in the case of vision. The information-preserving transformation from external world to internal representation is quite different in these two cases, and different again in the other modalities.

Proprietary patterns of activation across the cellular populations of your many other sensory modalities complete the story of peripheral world-representation. Prima facie, there is nothing “prepositional” about any of these representations, either in their various “syntaxes” or in their diverse “semantics.”

These intricate patterns—or activation vectors, as they are called—are projected inwards from the periphery, along crowded axonal highways, to secondary cell populations within the brain called the primary sensory cortices, one for each of the sensory modalities. Here too, representation consists in the pattern of activation levels across the cortical population of neurons, patterns provoked by the arriving sensory vectors.

But the patterns at this level are not mere repetitions of the original patterns at the sensory periphery. Those patterns have been transformed during their journey to the cortical populations. They get transformed mainly by the vast filter of synaptic connections they have to traverse in order to stimulate the cortical population. The result is typically a new pattern across the cortical canvas, a principled transformation of the original sensory pattern.

Such transformations illustrate the second major idea of the new approach. Computation over these vectorial representations consists in their principled transformation by the vast matrix of tiny synaptic connections that intervene between any two neuronal populations. Such a process, note well, performs a prodigious number of elementary computations all at once, since each of the

(possibly 10 12 ) synaptic connections does its job at the same time as all the other connections in the same matrix. This is called “massively parallel processing” and it provides us with a robust explanation of how animals can perform their extraordinary feats of computation in real time despite having “wetware” that is millions of times slower than the electronic hardware of conventional computers.

An intuitive way to think of such transformations is as follows. Consider a pictorial image projected through a nonuniform lens, or reflected from a deformed mirror. The image that comes out is quite different from the image that went in. And by configuring the surface of the lens/mirror to suit our purposes, we can produce any general transformation in the image we desire. Here the input

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incidentally, consists in modifying the configuration of synaptic connections. Learning, in other words, modifies the way we transform patterns.

The several cortical populations project in turn to further cell populations, and those to populations further still, until eventually the receiving population consists of motor cells, cells whose patterned activity is transformed by the muscle spindles into coherent bodily movement of some kind. Thus do we complete the basics of our new conception of how the nervous system works, from perception through cognition to organized behavior. It is here a stick-figure portrait, to be sure, but you will find it richly articulated in many directions in the literature. Patricia Churchland’s and Terry Sejnowski’s (1992) book provides an accessible and richly illustrated entry into the current state of research. My (1989) book attempts to draw out some of its consequences for epistemology and the philosophy of science.

What is important for the issues of this paper is that the relevant sciences have indeed articulated fertile and systematic theories concerning representation and computation in the brain. From the perspective of those theories, the most general and fundamental form of representation in the brain has nothing discernible to do with propositions, and the most general and fundamental form of computation in the brain has nothing discernible to do with inferences between propositions. The brain appears to be playing a different game from the game that FP ascribes to it.