Re: Rosch: Categorisation

From: Smith, Wendy (
Date: Mon Dec 04 1995 - 09:11:32 GMT

The purpose of categorisation is to ease the cognitive burden for
making decisions in the world, without reducing the information to
the extent that wrong decisions are made. By categorising,
everything within a category can be treated in the same way, and
different from everything in another category. Therefore, properties
relevant to the task in hand must be used to define the category, and
irrelevant properties ignored. The real world possesses structure,
which can be used to this end. For example, certain properties tend
to occur together, whereas other combinations are rare, eg scales are
associated with fins, and feathers with wings.

ER> ...our segmentation of a bird's body such that there is an
ER> attribute called "wings" may be influenced not only by perceptual
ER> factors...... but also by the fact that at present we already have a
ER> cultural and linguistic category called "birds".

I didn't quite follow this argument. Does she mean that because we have
a category labelled "birds" we are more likely to consider wings as a
relevant property? But don't the relevant properties determine the
categories? Is it that there are naturally occurring categories in the
real world, and the categories in the perceived world follow this
structure? Is this the "carving at the joints" bit?

Categories can be described in both the horizontal and vertical
dimension. The vertical dimension gives increasing abstraction one way,
and increasing specificity of properties the other way. The horizontal
dimension gives equivalent categories, defined by prototypes.

There is a basic level in the vertical hierarchy, which has
significantly more attributes than the superordinate category, but not
significantly less than the subordinate category. The basic categories
also had motor movements and shapes in common. Basic categories were
also the most abstract level at which a representative image could be
formed, and the one at which objects were perceived or recognised,
learned and named.

Again, I didn't quite follow this argument. Is she saying that the
basic categories are the basic categories because our imagery,
perceptions, language etc all converge at this level? Or is it because
we learn these as a basic category that our perceptions, imagery,
language etc use this as a base?

Categories can be defined either by their centres (ie the most typical
member) or by their boundaries. Prototypes can be agreed even when
boundaries are hazy (can this also go the other way - ie hazy prototype
and clear boundary? I'm thinking of "games" here). I though a
prototyp[e was something like the "ideal" category member, but that
doesn't seem the impression given here. So, what exactly is a

The more properties a member shares with the prototype, or other
members of the same category, the more prototypical it is. Conversely,
prototypicality is negatively correlated with the number of properties
it shares with member of different categories. It is also related to
various psychological measurements, eg reaction time, speed of learning
etc. However, prototypes alone are not enough to provide a model of
categorisation or an explanation of how categories are represented.
Furthermore, "a" prototype is only possiblewith artificially designed
categories (because it can be specified, property by property, by the
experimenter), and hence in the real world a prototype may be somewhat

There are problems. Some of the properties, rather than establishing a
prototype for a category, can only be assigned after the category
system has been developed - eg large for piano. Yes, it's large for a
piece of furniture, but that implies that the category "furniture" has
already been established. When participants were rating prototypicality
of category members, their judgement appeared affected by their
knowledge of the categories. In other words, they were demonstrating
C.P.! Prototypes will also be context dependent, in that if the
prototype is for the category "bird", it will differ depending upon
whether the context is "African" or "English". If a context is not
specified, people will supply their own - possibly the most usual
context for them.

It would also appear that events over the course of time can be chunked
into basic units, which are fairly consistent across people, and also
across distant and recent memories. The units tend to be those which
have a script. Again, I was confused here - couldn't this mean that the
unit may have a cultural basis, which could explain the consistency?
Anyway, one of the ways in which a boundary was defined in these events
was by the objects that a person was interacting with. So, for example,
there were "getting dressed" objects - jeans, t-shirt, socks, in one
event, and "having breakfast" objects in the next event - coffee,
cereal, kettle.

The objects which are most prototypical for these categories are those
which fit best into the scripts for the categories, eg cereal is a
better fit and more prototypical for "having breakfast" than steak and
chips. Again, I'm not sure of the point ehre - if both are learned
within a culture, then the more prototypical will fit into the script
better than the less prototypical, won't they? Also, if the change in
categories appears to signal a change in behaviour, ie heralds a
different event, then doesn't this make the boundaries important?

Anyway, within a narrtive for a n event, basic level objects are
usually used, and provide the best understanding. Superordinate levels
are too abstract, and subordinate levels are uneconomical and appear

I found the emphasis on introspective evidence in this article
confusing, because I couldn't disentangle which came first - a bit
chicken and egg! However, in summary, each category is typified by a
prototype, and inclusion in the category depends upon the number of
properties an item shares with the prototype. The properties are those
relevant to a particular context, or there would be an ugly duckling
situation. The boundaries of the categories do not have to be clear cut
to define and maintain the category. Prototypes may also be involved in
the learning of categories. Although in real life the prototype is
considered to be rather abstract, the examples given in the article
were concrete, and prototypes didn't seem to work as well for the more
abstract, superordinate levels.


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