But how exactly does the analogy work? The following three claims summarize Fodor’s distinctive way of working out the computer model of the mind.
1 Causation through content is ultimately a matter of causal interactions between physical states.
2 These physical states have the structure of sentences and their sentence-like structure determines how they are made up and how they interact with each other.
3 Causal transitions between sentences in the language of thought respect the rational relations between the contents of those sentences in the language of thought.
The second and third claims represent Fodor’s distinctive contribution to the problem of causation by content. The second is his influential view that the medium of cognition is what he calls the language of thought. According to Fodor, we think in sentences, but these are not sentences of a natural language such as English. The language of thought is much more like a logical language, such as the propositional calculus (which we looked at briefly earlier in this chapter– seeBox 6.1). It is supposed to be free of the ambiguities and inaccuracies of English.
The analogy between the language of thought and logical languages is at the heart of Fodor’s solution to the problem of causation by content. It is what lies behind claim (3). The basic fact about formal languages that Fodor exploits is the clear separation that they afford between syntax and semantics.
Consider, for example, the predicate calculus. This is a logical language more powerful and sophisticated than the propositional calculus we looked at in Box 6.1. Unlike the propositional calculus (which only allows us to talk about complete sentences or prop- ositions) the predicate calculus allows us to talk directly about individuals and their properties. In order to do this the predicate calculus has special symbols. These special symbols include individual constants that name particular objects, and predicate letters that serve to name properties. The symbols are typically identifiable by simple
typographical features (such as upper case for predicate letters and lower case for individual constants) and they can be combined to make complex symbols according to certain rules.
Viewed syntactically, a formal language such as the predicate calculus is simply a set of symbols of various types together with rules for manipulating those symbols according to their types. These rules identify the symbols only in terms of their typographical features. An example would be the rule that the space after an upper-case letter (e.g. the space in“F—”) can only be filled with a lower-case letter (e.g. “a”). Simplifying somewhat, this rule is a way of capturing at the syntactic level the intuitive thought that properties apply primarily to things– because upper-case letters (such as “F—”) can only be names of properties, while lower case letters (such as“a”) can only be names of objects. The rule achieves this, however, without explicitly stating anything about objects and properties. It just talks about symbols. It is a matter purely of the syntax of the language.
The connection between the formal system and what it is about, on the other hand, comes at the level of semantics. It is when we think about the semantics of a formal language that we assign objects to the individual constants and properties to the predicates. We identify the particular object that each individual constant names, for example. To provide a semantics for a language is to give an interpretation to the symbols it contains– to turn it from a collection of meaningless symbols into a representational system.
Just as one can view the symbols of a formal system both syntactically and semantic- ally, so too can one view the transitions between those symbols in either of these two ways. The predicate calculus typically contains a rule called existential generalization. This rule can be viewed either syntactically or semantically. Viewed syntactically, the rule states that if on one line of a proof one has a formula of the form Fa, then on the next line of the proof one can write the formula∃x Fx.
Viewed semantically, on the other hand, the rule states that if it is true that one particular thing is F then it must be true that something is F. This is because the expression“∃x Fx” means that there is at least one thing (x) that is F – the symbol “∃” is known as the existential quantifier. All transitions in formal systems can be viewed in these two ways, either as rules for manipulating essentially meaningless symbols or as rules determining relations between propositions.
Exercise 6.6 Explain the distinction between syntax and semantics in your own words. It is because of this that it is standard to distinguish between two ways of thinking about the correctness of inferential transitions in formal systems. From a syntactic point of view the key notion is logical deducibility, where one symbol is derivable from another just if there is a sequence of legitimate formal steps that lead from the second to the first. From the semantic point of view, however, the key notion is logical consequence, where a conclusion is the logical consequence of a set of premises just if there is no way of interpreting the premises and conclusion that makes the premises all true and the conclusion false. We have logical deducibility when we have a derivation in which every step follows the rules, while we have logical consequence when we have an argument
that preserves truth (that is, one that can never lead from a true premise to a false conclusion).
Fodor’s basic proposal, then, is that we understand the relation between sentences in the language of thought and their content (or meaning) on the model of the relation between syntax and semantics in a formal system. Sentences in the language of thought can be viewed purely syntactically. From the syntactic point of view they are physical symbol structures composed of basic symbols concatenated according to certain rules of composition. Or they can be viewed semantically in terms of how they represent the world (in which case they are being viewed as the vehicles of propositional attitudes). And so, by extension, transitions between sentences in the language of thought can be viewed either syntactically or semantically– either in terms of formal relations holding between physical symbol structures, or in terms of semantic relations holding between states that represent the world.