Minds and Machines

1. Minds and Machines Summer 2011 Tuesday, 8/2

2. 608. No supposition seems to me more natural than that there is no process in the brain correlated with associating or with thinking; so that it would be impossible to read off thought-processes from brain-processes. I mean this: if I talk or write there is, I assume, a system of impulses going out from my brain and correlated with my spoken or written thoughts. But why should the system continue further in the direction of the centre? Why should this order not proceed, so to speak, out of chaos? . . . 609. It is thus perfectly possible that certain psychological phenomena cannot be investigated physiologically, because physiologically nothing corresponds to them.

3. Connectionism and Folk-Psychology • Stich, Ramsey and Garon argue that if (a certain version of) Connectionism is true, then we must be Eliminativists about propositional attitudes. • They first argue that if Connectionism is true, we must either be Eliminativists or Reductivists about propositional attitudes. (The theory change must either be “conservative” or “radical”) • Given that there is nothing, in a connectionist model, that we could plausibly identify propositional attitudes (e.g. beliefs) with, the truth of connectionism forces us to deny any sort of reality to the attitudes (or to make a “radical” theory change).

4. Connectionism and Folk-Psychology • Propositional Modularity (Fodor’s “essence of the attitudes”): the idea that our ordinary use of propositional attitude talk involves a commitment to discrete, semantically interpretable states that play a causal role in the production of other mental states and behavior. • Reasons to accept it: (1) our talk of gaining and loosing beliefs one at a time, (2) our explanations of people’s behavior involve citing specific beliefs as causes.

5. Connectionism and Folk-Psychology Properties of Connectionist Models: • Their encoding of information in the connection weights is widely distrubuted. • Individual hidden units in the network have no comfortable symbolic interpretation. The networks are not semantically transparent, but are sub-symbolic. Implementational vs. Cognitive connectionism.

6. Connectionism and Folk-Psychology Reasons to think that Connectionism is incompatible with Propositional Modularity. Consider a connectionist network that’s trained to give “yes”/”no” answers to a set of 16 questions, e.g. “do dogs have fur?”, “do fish have fur?”, etc.

7. Connectionism and Folk-Psychology • “In the connectionist network, there is no distinct state or part of the network that serves to represent any particular proposition. The information encoded in the network is stored holistically and distributed throughout the network. Whenever information is extracted from the network, by giving it an input string and seeing whether it computes [a yes or no answer] for the output unit, many connection strengths…many hidden units play a role in the computation. And any particular weight or unit will help to encode information about many different propositions. It simply makes no sense to ask whether or not the representation of a particular proposition plays a causal role in the network’s computation. It is in just this respect that our connectionist model seems radically incongruent with the propositional modularity of common sense psychology.” (Stich, et el)

8. Connectionism and Folk-Psychology 2. Consider what happens when we compare the original network with one that involves an additional piece of knowledge. Intuitively, we want to say that the two networks share many of the same beliefs. Yet the two nets may have no subset of weights in common! Their commonality is invisible at the level of units and weights. But if there were some discrete states in the connectionist model that played the role of beliefs, we would expect there to be lots of commonalities. [1] and [2] suggest that there are no discrete, semantically evaluable and causally potent states in a connectionist network that could plausibly be identified with beliefs. So if connectionism is the correct model of the mind, we must deny that the mind really contains entities like beliefs.

9. Connectionism and Folk-Psychology Objection 1: Connectionist models do not really violate propositional modularity, since the propositions the system has learned are coded in functionally discrete ways, though this may not be obvious. (Think of the way propositions are stored in a computer, at physically scattered memory addresses. Still, if one knew enough about the system, one could erase any one sentence by tampering with the contents of the relevant memory addresses.) Reply: This is a possibility, but at present, there is no reason to take it seriously.

10. Connectionism and Folk-Psychology Objection 2: The propositions are encoded in the patterns of activation of the hidden units, when a given question is presented to the network. Reply: Such patterns of activation are not enduring states of the network. So it’s implausible that they play the role of knowledge or beliefs.

11. Connectionism and Folk-Psychology Objection 3: Beliefs should not be identified with activation patterns (or any transient states) but with dispositions to produce activation patterns given certain inputs. Reply: Dispositions are enduring states, but they are not the right sorts of enduring states to be identified with beliefs. In particular, they do not seem to be capable of playing a causal role.

12. Connectionism and Folk-Psychology Other responses to Stich (et al) are: 1. Question the commitment to propositional modularity (Dennett). 2. Accept that if (cognitive) connectionism is true, then Folk Psychology is false, but argue that this conditional is a reason to reject connectionist models of the mind rather than Folk Psychology.

13. Systematicity Fodor would go with 2, but he also argues against connectionist models of (certain parts of) the mind like this: • Thought is systematic. • So internal representations are structured. • Connectionist models lack structured internal representations; C. Connectionist models are not good models of human thought.

14. Systematicity Responses: • Classical symbol systems are not the only way to support systematically structured cognition. (Problem with this response: neural networks are not particularly good at the kind of rule based processing that is thought to undergird language, reasoning, and higher forms of thought.) • Human thought may inherit its systematicity from the grammatical structure of language. (This reply downplays the extent and importance of systematicity, something that may be independently plausible).

15. Quiz • What are artificial neural networks and how do they differ from physical symbol systems? • What reason is there for thinking that (cognitive) connectionism implies eliminativism about propositional attitude psychology?