SEL2211: Contexts
Lecture 6: The computational theory of mind
Humans are conscious in the sense that each of us has an
awareness of selfhood.
We interact with the world using our senses and our capacity for
movement, and on the basis of that interaction we come to have
knowledge of and beliefs about the world, which in their turn motivate
further interaction.
In short, humans have minds. To understand what minds are, and
how they work, is to understand what it is to be human.
This lecture briefly outlines the history of the study of the mind, then
goes on to look at contemporary computational interpretations of mind
and language, and concludes with objections to these interpretations.
SEL2211: Contexts
Lecture 6: The computational theory of mind
1. Understanding the mind: a brief overview
From the earliest historical times, and presumably before that, humans have
wondered what it is that differentiates them from other animals --what makes
them human-- and this has led them to speculate on the nature of what we
now call the mind.
Evidence that this was so comes from all over the world.
For the purposes of this discussion, however, we will concentrate on the
Western tradition for the simple reason that it leads directly to contemporary
cognitive science, which is our object of interest.
SEL2211: Contexts
Lecture 6: The computational theory of mind
1. Understanding the mind: a brief overview
1.1 Ancient Greek philosophy
The earliest explicit and coherent Western account of the mind comes from the
Greek philosopher Plato (c. 428/7-348/7 BC), who proposed two separate worlds:
the physical world of human experience, and a world of non-physical forms or
ideas.
The world of ideas contains the eternal and unchanging forms of things, and the
world of experience is an imperfect and constantly-changing realization of those
forms.
Human souls (or, in modern terms, minds) originally dwelt in this ideal world, and
can regain knowledge of it.
This cannot be done solely on the basis of physics, the study of the world of
experience, since the objects of study are imperfect realizations of the forms,
though physics is useful in that it moves the mind to remembrance of the ideas it
once knew.
SEL2211: Contexts
Lecture 6: The computational theory of mind
1. Understanding the mind: a brief overview
1.2 Rome and the Middle Ages
Plato's thinking on the soul was hugely influential, both during the period of the
Roman Empire (c.50 BC - c.500 AD), and in the Middle Ages (c.500 - c.1500 AD).
Plato's affirmation of a spiritual order of reality fit well with the Christian view that
the spiritual aspect of humanity iss of primary importance in life.
The great majority of Christian philosophers until the thirteenth century were
Platonists, recognizing in Platonism a powerful ally in the fight against materialism.
Thereafter the teachings of Plato's pupil Aristotle (384-322 BC) became dominant.
Aristotle challenged the Platonic doctrine of ideals, and paved the way for postmedieval thinking about the mind, but the idea that the soul/mind was a nonphysical substance, a piece of divinity which enters the body at conception and
leaves it on death, has remained fundamental to Christian thought.
SEL2211: Contexts
Lecture 6: The computational theory of mind
1. Understanding the mind: a brief overview
1.3 The West since c.1500 AD
As a consequence both of Aristotelian thought and of the rediscovery of Ancient
Greek and Roman texts, Europe after about 1500 AD entered a period of
intellectual revolution now known as the Renaissance.
The main characteristics of the Renaissance were, on the one hand, a tendency to
denigrate religious ideology, and on the other to promote scientific investigation,
which was understood as the empirical investigation of nature.
This led to rapid advances in such areas as mathematics, astronomy, and physics,
together with the development of technology based on the new sciences.
SEL2211: Contexts
Lecture 6: The computational theory of mind
1. Understanding the mind: a brief overview
1.3 The West since c.1500 AD
The mind now came to be seen not as more or less equivalent to a divine soul, but
as something rooted in the physical operation of the brain, and thus amenable to
study just like any other aspect of the physical world.
As scientific thought and technology progressed, the mind was seen successively
as a hugely complex clockwork, as a hydraulic engine, and, by the end of the
nineteenth century, as a something like a telephone exchange.
In the second half of the 20th century, it came to be seen as a computer.
SEL2211: Contexts
Lecture 6: The computational theory of mind
2. The computational mind
2.1 The sciences of the mind
Turing himself proposed that the human mind was a computer. This suggestion
was enthusiastically taken up by a variety of scientific disciplines in the 1950s:
-- Cognitive science, which attempts to explain the operation of the human mind.
-- Generative linguistics . Chomsky has always seen the linguistics he pioneered as
an aspect of psychology whose remit was to understand a specific human cognitive
function: language.
-- Artificial Intelligence, an engineering discipline which attempts to construct
computational systems which display aspects of human cognitive capacity, and
whose ultimate aim is creation of machines with general human-level intelligence.
Since then, Turing’s hypothesis that the human mind is a computer has been
dominant in these disciplines.
SEL2211: Contexts
Lecture 6: The computational theory of mind
2. The computational mind
2.2 The architecture of the computational mind
How does the computational mind work? Like a very complex Turing machine:
SEL2211: Contexts
Lecture 6: The computational theory of mind
2. The computational mind
2.2 The architecture of the
computational mind
On this model, the senses and
muscles are the input and output
tapes of the Turing Machine, and
there are numerous algorithms to
control every aspect of cognitive
functioning: logic, memory,
perception, movement, and
language.
In each case, of course, the
algorithm must be extremely
complex to account for human
behaviour, but the principle
remains the same as for the very
simple Turing Machine we looked
at earlier.
SEL2211: Contexts
Lecture 6: The computational theory of mind
2. The computational mind
2.3 Mental algorithms
What does a cognitive algorithm look like?
The truth is that, in the present state of cognitive science, no one knows for sure,
but we do have some reasonable theories.
One such theory is based on the Language of Thought (henceforth LOT)
hypothesis, which proposes that thinking is done in an internal language which has
come to be known as 'mentalese‘.
Assuming this to be the case, algorithms for the various cognitive functions would
specify how the sentences of mentalese are constructed and interpreted.
This section considers what a language is, then what a language of thought might
look like, and finally what form an algorithm for the language of thought might take.
SEL2211: Contexts
Lecture 6: The computational theory of mind
2. The computational mind
2.3 Mental algorithms
2.3.1 Language
What is a language?
From a computational point of view, a language is just a set of strings, that is, a set
of symbol sequences where the symbols are taken from a symbol system.
Some examples (the brackets {...} denote a set):
L1 = {heajk, hf, hju, mh, hfrqiqjhi}
L2 = {y=ax+b, y=ax2+bx+c, y=ax3+bx2+cx+d}
L3 = {the cat sat on the mat, the man ran}
SEL2211: Contexts
Lecture 6: The computational theory of mind
2. The computational mind
2.3 Mental algorithms
2.3.1 Language
L1 = {heajk, hf, hju, mh, hfrqiqjhi}
L2 = {y=ax+b, y=ax2+bx+c, y=ax3+bx2+cx+d}
L3 = {the cat sat on the mat, the man ran}
L1 consists of strings of symbols taken from the Western alphabet. The strings are
gobbledegook, but that doesn't matter: a language is defined as a set of strings, as
this is, without reference to what the strings might mean.
L2 is a set of strings of symbols taken from the mathematical symbol system.
L3 is a set of strings of symbols taken from the symbol set of English words. These
and other languages can contain as many strings as one likes --natural languages,
for example, are unbounded, as is the set of possible mathematical expressions-but the principle is always the same: a language is a set of strings.
SEL2211: Contexts
Lecture 6: The computational theory of mind
2. The computational mind
2.3 Mental algorithms
2.3.1 Language
A language can consist of all possible combinations of all the symbols in the
associated symbol system.
In English, for example, one could have strings like the the the the the or rest bad
walks the already, and so on.
In general, though, there are constraints on which strings belong to any given
language and which do not: the English word sequences just cited don't belong to
the English language, but Bill ran home and I won the lottery do.
SEL2211: Contexts
Lecture 6: The computational theory of mind
2. The computational mind
2.3 Mental algorithms
2.3.1 Language
How can one specify which strings belong to any given language and which does
not?
By establishing a set of rules by which the strings that belong to the language may
be formed; all strings not constructed according to the rules are excluded from the
language.
Such sets of rules are known as grammars.
There are various ways of stating grammars, but one familiar to linguists is rewrite
rules.
SEL2211: Contexts
Lecture 6: The computational theory of mind
2. The computational mind
2.3 Mental algorithms
2.3.1 Language
The following grammar generates (among others) the strings that constitute L3:
{the cat sat on the mat, the man ran}
S > NP VP
NP > DET N
VP > V
VP > V PP
PP > P NP
DET > {the set of English determiners}
N > {the set of English nouns}
V > {the set of English verbs}
P > {the set of English prepositions}
SEL2211: Contexts
Lecture 6: The computational theory of mind
2. The computational mind
2.3 Mental algorithms
2.3.1 Language
In this grammar there is one rewrite rule per line.
Rewrite rules are so called because the symbol on the left of the '>' can be
rewritten as the symbol of symbols on the right.
A string in L3 is constructed by starting with S and then rewriting it according to the
grammar.
For example:
SEL2211: Contexts
Lecture 6: The computational theory of mind
2. The computational mind
2.3 Mental algorithms
2.3.1 Language
S > NP VP
NP > DET N
VP > V
VP > V PP
PP > P NP
DET > {the set of English
determiners}
N > {the set of English nouns}
V > {the set of English verbs}
P > {the set of English
prepositions}
S
NP VP
DET N VP
the N VP
the cat VP
the cat V PP
the cat sat PP
the cats sat P NP
the cat sat on NP
the cat sat on DET N
the cat sat on the N
the cat sat on the mat
SEL2211: Contexts
Lecture 6: The computational theory of mind
2. The computational mind
2.3 Mental algorithms
2.3.1 Language
S > NP VP
NP > DET N
VP > V
VP > V PP
PP > P NP
DET > {the set of English
determiners}
N > {the set of English nouns}
V > {the set of English verbs}
P > {the set of English
prepositions}
S
NP VP
DET N VP
the N VP
the cat VP
the cat V PP
the cat sat PP
the cats sat P NP
the cat sat on NP
the cat sat on DET N
the cat sat on the N
the cat sat on the mat
Successive application of the grammar rules has generated a string of the language; the
man ran in L3 would be similarly generated, as would many other English strings.
SEL2211: Contexts
Lecture 6: The computational theory of mind
2. The computational mind
2.3 Mental algorithms
2.3.1 Language
Generating strings in accordance with a grammar of this sort endows them with
constituency structure, which can be represented graphically as a tree:
A string, therefore, is a structured object, that is, and object consisting of parts which
may themselves consist of parts which may themselves consist of parts, and so on.
SEL2211: Contexts
Lecture 6: The computational theory of mind
2. The computational mind
2.3 Mental algorithms
2.3.1 Language
The structuredness of strings is of
fundamental importance in how
language algorithms work.
To see why, consider the string the saw
the man with a telescope. This string is
ambiguous.
There's no way to tell which meaning is
intended from the string alone, but the
combination of string and structure
allows it to be disambiguated.
SEL2211: Contexts
Lecture 6: The computational theory of mind
2. The computational mind
2.3 Mental algorithms
2.3.1 Language
A language algorithm both endows a string which structure in the course of
generating it, and uses the structure associated with a string in the course of
interpretation.
SEL2211: Contexts
Lecture 6: The computational theory of mind
2. The computational mind
2.3 Mental algorithms
2.3.2 The Language of Thought
The language of thought proposes that the various aspects of cognition are
generated by algorithms operating on structured symbol strings in a way that is
sensitive to their structure.
What do these strings look like?
Again, no one knows for sure at the present time, but given that symbols are
physical, one plausible suggestion is that mental symbols in the LOT are neural
ensembles, that is, assemblies of neurons whose connections cause them to
behave as a group and thereby to constitute a symbol.
SEL2211: Contexts
Lecture 6: The computational theory of mind
2. The computational mind
2.3 Mental algorithms
2.3.2 The Language of Thought
In the following graphic, for example, each ensemble has a different pattern of
connectivity and therefore a different firing pattern; each can be taken to be a
symbol in the LOT, and a string would be a sequence of different patterns of
activation over time.
SEL2211: Contexts
Lecture 6: The computational theory of mind
2. The computational mind
2.3 Mental algorithms
2.3.3 Algorithms of the Language of Thought
The algorithms of the LOT are almost totally mysterious.
We know how neurons and neural ensembles work, and we can observe them
working by means of the scanning techniques mentioned in the previous lecture,
but we are not yet in a position to know what the pattern of activity we observe is
doing, that is, we don't yet know the brain's algorithms.
Some animations:
Youtube. MRI Brain activity: http://www.youtube.com/watch?v=H-De5nOC8K0
Youtube. Spontaneous brain
activity: http://www.youtube.com/watch?v=3nCBLw9Z-xU&feature=related
Youtube. Music processing:
http://www.youtube.com/watch?v=FVIiYyD2_YU&feature=related
SEL2211: Contexts
Lecture 6: The computational theory of mind
2. The computational mind
2.4 The Language of Thought and Natural Language
Proponents of the cognitive view have included such luminaries as Wilhelm von
Humboldt, Vygotsky, Wittgenstein, and Daniel Dennett. The idea that thought and
language are causally interrelated is, however, primarily associated with the
American linguists Edward Sapir and his pupil Benjamin Whorf, whose stance on
this idea is best summed up by a much-cited quotation from Whorf:
“We dissect nature along lines laid down by our native languages. The categories
and types that we isolate from the world of phenomena we do not find there
because they stare every observer in the face; on the contrary, the world is
presented in a kaleidoscopic flux of impressions which has to be organized by our
minds - and this means largely by the linguistic systems in our minds. We cut
nature up, organize it into concepts, and ascribe significances as we do, largely
because we are parties to an agreement to organize it in this way - an agreement
that holds throughout our speech community and is codified in the patterns of our
language. The agreement is, of course, an implicit and unstated one, but its terms
are absolutely obligatory; we cannot talk at all except by subscribing to the
organization and classification of data which the agreement decrees.”
SEL2211: Contexts
Lecture 6: The computational theory of mind
2. The computational mind
2.4 The Language of Thought and Natural Language
On the basis of this and other passages in the various writings of Sapir and Whorf,
the linguistics research community has constructed the Sapir-Whorf hypothesis,
which proposes two associated principles:
-- Linguistic determinism, whereby thinking is determined by language – the
language that a person speaks determines the way he or she interprets the world.
-- Linguistic relativity, whereby people who speak different languages perceive and
think about the world differently.
The communicative view – that language is purely a mechanism for the
communication of independently-existing thought – is based on the computational
model of the mind. It was held by, among others, John Locke and Bertrand Russell,
and has been standard in cognitive science in the second half of the twentieth
century.
SEL2211: Contexts
Lecture 6: The computational theory of mind
2. The computational mind
2.4 The Language of Thought and Natural Language
The standard communicative view of the relationship between thought and
language is now being challenged, most recently by Dennet and Carruthers.
Both argue that the cognitive view is no less inherently plausible than the
communicative one, and that, like the latter, it has substantial empirical support.
Carruthers goes on to propose that language is only involved in a specific type of
thought – conscious thought.
This, of course, begs the question of what a conscious thought might be, but a
rough approximation is that it includes the traditionally ‘higher’ functions like
reasoning, planning, and memory, and excludes ‘lower’ ones like perceptual and
motor functions.
SEL2211: Contexts
Lecture 6: The computational theory of mind
3. Objections to the computational theory of mind and language
In the late 1980s, the consensus that human minds, and language more
specifically, are computational systems began to be questioned on a variety of
grounds, both theoretical and practical.
We will look at the main theoretical grounds here.
.
SEL2211: Contexts
Lecture 6: The computational theory of mind
3. Objections to the computational theory of mind and language
3.1 The essential problem: symbols, meaning, and the mind
At the root of the theoretical objections to the computational mind is the nature of
symbols.
Symbols are arbitrary relative to that which they represent: the meaning of a
symbol –what is represents- is in the mind of the human who interprets it.
Why is this crucial to the hypothesis that the mind is a computer?
The argument is as follows:
.
SEL2211: Contexts
Lecture 6: The computational theory of mind
3. Objections to the computational theory of mind and language
3.1 The essential problem: symbols, meaning, and the mind
i.
Computers manipulate symbols, but they do not themselves interpret them –they
do not know what the symbols they manipulate mean.
Interpretation of a computer’s response to input is in the mind of the human
observing it.
A computer does not know the meaning of an essay typed in from a keyboard, nor
does it secretly read emails, nor does it take pleasure in space invaders games.
The meanings of these things arise in the mind of a human in response to
observation of the monitor display.
SEL2211: Contexts
Lecture 6: The computational theory of mind
3. Objections to the computational theory of mind and language
3.1 The essential problem: symbols, meaning, and the mind
ii.
There is no meaning in a computer, but there IS meaning in a human mind.
A computational explanation of cognition on its own is therefore insufficient: it
cannot explain how meaning arises in the mind.
SEL2211: Contexts
Lecture 6: The computational theory of mind
3. Objections to the computational theory of mind and language
3.1 The essential problem: symbols, meaning, and the mind
The lack of meaning in a computational system is analyzed as the 'symbol
grounding problem': a computational system consists of symbols and symbol
manipulation, and only these things, and cannot by definition incorporate that which
the symbols represent.
SEL2211: Contexts
Lecture 6: The computational theory of mind
3. Objections to the computational theory of mind and language
3.2 Proposed solutions
Researchers have taken different approaches to the solution of the symbol
grounding problem.
-- Some, wishing to retain the traditional computational model of mind, have argued
that meaning somehow arises from the interaction of the various computational
modules of mind, though it is difficult to see how this could be.
SEL2211: Contexts
Lecture 6: The computational theory of mind
3. Objections to the computational theory of mind and language
3.2 Proposed solutions
-- Others have suggested that not all of cognition is computational -that only certain
modules and perhaps only language are.
In particular, the brain's processing of sensory and motor input/output is not
symbolic, and thus not computational.
By associating linguistic symbols with such non-symbolic brain processes,
language can be grounded in bodily experience of the world, and this gives rise to
meaning in the cognitive system as a whole.
SEL2211: Contexts
Lecture 6: The computational theory of mind
3. Objections to the computational theory of mind and language
3.2 Proposed solutions
Thus, for example, the word ‘cat’ means our experience of cats in the world in
terms of how they look, sound, behave, and so on; once word meanings are
grounded in experience in this way, sentence meanings can be built up
compositionally by symbol manipulation.
This development of computational cognitive science is called ‘embodied cognition’,
and is currently being intensively developed by psychologists and cognitive science
researchers, though it has so far failed to make any real impact in generative
linguistics
SEL2211: Contexts
Lecture 6: The computational theory of mind
3. Objections to the computational theory of mind and language
3.2 Proposed solutions
-- Still other researchers have concluded that minds, including the language faculty,
are not computational, and have proposed alternative theoretical frameworks for
understanding cognition and language.
These alternative approaches stress the study of brain mechanisms using MRI
scanning technology, and the mathematical theory of complex systems, which
treats brains and their cognitive functions in the same terms as other highly
complex natural systems like the cosmos, the weather, and biological ecologies.
Some psychologists and artificial researchers are taking this approach, but once
again generative linguists show no signs of doing so.