Goal-Driven Conceptual Blending: A Computational Approach for
Creativity
 Boyang Li, Alexander Zook, Nicholas Davis and Mark O. Riedl
School of Interactive Computing, Georgia Institute of Technology, Atlanta GA, 30308 USA
{boyangli, a.zook, ndavis35, riedl}@gatech.edu
Abstract
Conceptual blending has been proposed as a creative
cognitive process, but most theories focus on the
analysis of existing blends rather than mechanisms for
the efficient construction of novel blends. While
conceptual blending is a powerful model for creativity,
there are many challenges related to the computational
application of blending. Inspired by recent theoretical
research, we argue that contexts and context-induced
goals provide insights into algorithm design for creative
systems using conceptual blending. We present two
case studies of creative systems that use goals and
contexts to efficiently produce novel, creative artifacts
in the domains of story generation and virtual
characters engaged in pretend play respectively.
 Introduction
Conceptual blending has been proposed as a fundamental
cognitive process, responsible for the creation of a broad
range of creative artifacts. (Fauconnier and Turner 1998,
2002; Grady 2000; Hutchins 2005). Fauconnier and Turner
(1998, 2002) proposed that conceptual blending involves
the merger of two or more input spaces into a blended
space. A griffin, for example, may be considered as a blend
of an eagle and a lion. Each of the input spaces contains a
number of concepts and their inter-connections. These
concepts are selectively merged and projected into the
blend space. After that, additional structures may emerge
in the blend by pattern completion and further elaboration.
 Among artifacts created by conceptual blending, we
distinguish two types of blends, namely semiotic
expressions and standalone concepts. Semiotic expressions
are used in communication to highlight certain aspects of
or shed light on one of the input spaces. An often discussed
semiotic expression is "this surgeon is a butcher" (cf.
Grady, Oakley, and Coulson 1999; Brandt and Brandt
2002; Veale and O'Donoghue 2000). The input spaces of
the expression are the conceptual spaces of the surgeon and
the butcher respectively. The surgeon in the blend
possesses the brutal attitude of the butcher, forming a
criticism of the surgeon.
 This paper focuses on the construction of the second
type of blend, which we call standalone concepts. An
example is the lightsaber from Star Wars: a lightsaber
blends together a sword and a laser emitter, but it is an
independent concept that does not inform hearers about the
properties of swords or laser emitters. During blend
creation, contents are still projected from the input spaces
into the blend, but the blend is not meant to convey
information about the input spaces.
 We share the belief that theories of creativity should be
computable (Johnson-Laird 2002), yet most accounts of
blending focus on analyses of existing blends and have not
fully described how novel blends are constructed
cognitively or algorithmically. In particular, three key
procedures required for blending lack sufficient details
necessary for efficient computation: (1) the selection of
input spaces, (2) the selective projection of elements of
input spaces into the blend, and (3) the stopping criteria for
blend elaboration. Inefficiencies in these procedures can
lead to significant difficulties in finding appropriate blends
and elaborations. For example, a simplistic algorithm may
produce all possible combinations of elements from all
input spaces, resulting in a combinatorial explosion of
possible blends.
 We argue that these three main procedures must
algorithmically make use of the context and goals of the
blend being constructed. Brandt and Brandt (2002)
proposed communication contexts and goals as the driving
force behind the three key procedures, but their analysis is
limited to semiotic expressions. We extend their theory to
the construction of novel standalone concepts and provide
computational justifications through two case studies of
working computational systems in the domains of story
generation and pretend play. Our systems construct blends
in a goal-driven and context-driven manner. As integral
aspects of the conceptual blending process, contexts and
goals provide concrete computational benefits by pruning
search spaces and improving average-case performance.
The Theories of Conceptual Blending
This section reviews the theories of conceptual blending
described by Fauconnier and Turner (1998, 2002) and
Brandt and Brandt (2002). We compare these two accounts
side by side and identify some underspecified parts in the
theories, which a working system must address.
International Conference on Computational Creativity 2012 9
 In the original blending theory (BT) by Fauconnier and
Turner (1998, 2002), conceptual blending takes two or
more mental spaces as inputs. Mental spaces are
dynamically constructed during a discourse (e.g.
conversation) to contain relevant concepts. The input space
of the surgeon, for example, includes the surgeon and
relevant entities, such as his scalpel, patient and so on.
Elements in one input space are then mapped to their
counterparts in another input space, using mapping rules
such as identity or analogy. In the surgeon-as-butcher
example, the cleaver of the butcher is mapped to the
scalpel of the surgeon; the dead animal is mapped to the
patient, and so forth. Elements from input spaces are
selectively projected into a blend space. The generic space
captures the structural similarities between input spaces. In
addition to elements projected from input spaces, the blend
space can also contain emergent structures created by
pattern completion or elaboration. This four-space
formulation is shown in Figure 1, where big circles denote
mental spaces, black dots represent elements in the spaces,
solid lines are the mappings between inputs, and dashed
lines denote correspondences among the elements in the
four spaces. Hollow dots denote emergent structures in the
blend.
 Fauconnier and Turner, however, did not specify how
elements from these input spaces could be chosen during
the selective projection. Although eight optimality
principles—human scale, topology, pattern completion,
integration, vital relations, unpacking, web, and
relevance—were proposed as quality measures, they can
only evaluate the quality of a complete blend after it is
constructed. This suggests a computational approach where
all possible blends have to be generated and tested
individually, called a neo-Darwinian algorithm by
Johnson-Laird (2002). A neo-Darwinian algorithm could
lead to a combinational explosion of options and is
infeasible for large input spaces. In contrast, what JohnsonLaird
calls a neo-Lamarckian approach generates only
valuable products by applying quality constraints on the
search space. To do so in blending requires a mechanism
that selects elements from input spaces effectively during
the generation process. Note that projection occurs after
inter-space mappings are built, so the complexity of
analogy making should not be conflated with the
complexity of projection.
 Moreover, BT does not provide detailed procedures for
the effective retrieval of input spaces, nor for the
elaboration of blends. A full computational implementation
of the blending theory should select input spaces by itself,
rather than assume them as given. To create a powerful
criticism of the surgeon, a system should decide to blend it
with a butcher, rather than a driver or a school teacher. A
creative system may possess a huge amount of knowledge,
so an efficient selection procedure for input spaces is
necessary for it to operate within reasonable time limits.
The same argument goes for elaboration. Neither a human
nor a computational system should elaborate a blend
endlessly. We usually do not wish to simulate the entire
world’s reaction to an irresponsible surgeon, which will
require excessive computational power or time.
 In summary, the original blending theory left much
ambiguity in three key procedures: (1) the selection of
input spaces, (2) the selection of elements for projection,
and (3) the stopping criterion for blend elaboration.
However, a complete working implementation of BT must
contain these procedures.
Context-Dependent Blending
Brandt and Brandt (2002) pointed out that blends used in
communication do not have fixed meanings. Rather, they
are real-world phenomena that can only be analyzed in the
context of the discourse during which they were uttered.
Under different circumstances, the same utterance “this
surgeon is a butcher” can mean different things. For
example, if a soldier referred to a battlefield medic as a
butcher, he may be highlighting the fact that he has to
perform an astonishing number of amputations. This
Figure 1. The four-space blending theory, adapted from
Fauconnier & Turner (2002).
Figure 2. The context-dependent blending theory, adapted from
Brandt and Brandt (2002).
International Conference on Computational Creativity 2012 10
interpretation is vastly different than the ethical judgment
about a surgeon’s skills after a failed surgery.
 To account for this plurality of meanings, blend
construction must not be based solely on the attributes of
the input domains, but on the context. Hence, Brandt and
Brandt abandoned the context-free generic space and
proposed a context-driven blend construction process. A
simplified version of Brandt and Brandt’s theory is shown
in Figure 2, where the dashed arrow indicates that the
situational relevance prompts the retrieval of the two input
spaces in order and solid arrows represent selective
projection. First, the communicative situation (context)
retrieves the input spaces in order. The situation implies a
first input space, the reference space based in the actual
real-world situation. For example, if we are talking about a
particular surgery, the input space of the surgeon becomes
available. Based on the goal of communication (e.g.
conveying that the surgeon is irresponsible), a presentation
space including the figurative entities (e.g. a butcher) is
then retrieved and mapped to the reference space. Second,
the goal of communication, captured through the
situational relevance, determines what elements from the
two spaces are projected into the blend. If we want to
accuse the surgeon of being irresponsible, we should
project the careless attitude of the butcher into the blend.
Third, elaboration of blends also depends on the context
and the goal, fleshing out a blended space until sufficient
detail is achieved for the guiding goal. In summary, the
sequential retrieval of input spaces, the selection of
projected elements, and the stopping criteria are all driven
by contexts and goals.
Blending Novel Concepts
In addition to metaphorical expressions, blending also
yields novel concepts independent of the input spaces, such
as the lightsaber. We believe the two cases differ in the
relationship between the inputs spaces and the blend.
 Blends used in communication are usually meant to
underscore certain aspects of, or to attach new properties to
an input space. In our example, the surgeon who possesses
a butcher’s attitude in the blend space becomes a criticism
of the surgeon in the input. The linkage between the blend
and the input spaces is essential for understanding.
 In contrast, standalone concepts are blends that are not
meant to convey meaning about their input spaces. For
example, the birth stork is a blend of the metaphor birth-isarrival,
air travel, and the stork (Fauconnier and Turner
2002, ch. 14), but it does not tell us much about air travel
or storks in general. As another example, the lightsaber in
Star Wars is clearly a blend of a sword and a laser emitter,
but it is not meant to tell us anything about swords or laser,
even though understanding laser and swords can help us
understand lightsabers. There are links going from the
input spaces to the blend spaces, but not vice versa.
 Standalone concepts created from blending are
commonly seen in stories and story-related activities.
Dragons, for example, possess features of snakes, large
cats and birds of prey, and the instinctive fear of the three
is postulated to be its origin (Jones 2002). Many other
mythical creatures are combinations of common animals.
Novel concepts created from blending also appear in
modern science fiction. The popular Japanese sci-fi manga
Doraemon (Fujio 1974-1996) contains several gadgets
made this way, such as a toy telephone that can transmit flu
instead of voice.
 The idea of an independent concept does not contradict
the notion of contextualized meaning. The meaning of a
novel concept, when used as a semiotic sign, can still
change depending on the context. One may say “my new
kitchen knife is a lightsaber” to emphasize its sharpness. In
Brandt and Brandt’s framework, this meaning is created
from the blend of the kitchen knife and the lightsaber,
which is a different blend than the lightsaber itself. The
concept lightsaber, on the other hand, is as independent as
the concept butcher. For another example, the birth stork is
now often a humorous symbol for baby births rather than
used to explain births. The contexts and meanings vary, but
the concepts remain relatively constant.
 We extend Brandt and Brandt’s contextualized blending
theory to the construction of novel concepts. Below we
present two computational systems that generate novel
concepts in fictions and pretend play. While Brandt and
Brandt’s (2002) theory initially was not meant to explain
such blends, we show that a goal and context driven
approach can account for their generation and bring
computational benefits.
Previous Implementations
Most previous work on computational algorithms for
conceptual blending are based on the theories of
Fauconnier and Turner and thus do not fully account for
input space selection, selective projection, or blend
elaboration. It is common for computational blending
algorithms to ignore one or more of these stages. The
Alloy blending engine, as part of the Griot poem generator
(Goguen and Harrell 2004), is the earliest implementation
of BT that we are aware of. Input spaces are manually
coded as symbolic expressions. Projection into the blend is
based on a structural mapping between input spaces.
Without the guidance of goals, any element from the input
spaces may be projected. The authors found the number of
possible blends is exponential to the number of relations in
the input spaces, but did not discuss methods to prune
these spaces. Hervás et al. (2006) proposed a process that
enriches texts of stories which can be considered as
conceptual blends. They proposed that readers’ familarity
with input spaces, as part of the communication context, is
important in selecting input spaces and elements for
projection without specifying an algorithm.
 Martinez et al. (2011) proposed a blending algorithm
where input spaces are sets of axioms. Compatible axioms
are selected for projection. The system does not consider
goals to directly build blends that meet specific purposes.
Rather, it "enumerates alternatives ranked by the
complexity of the underlying mappings". Yamada et al.
(2011) generate motion of dancing characters by
International Conference on Computational Creativity 2012 11
representing motion as wavelet equations and blending is a
weighted summation of wavelet coefficients. The process
does not involve any of the three key procedures
mentioned earlier. Thagard and Steward (2011) proposed
an implementation of blending at the neural level, which
also does not consider goals.
 The Divago system (Pereira 2007) is noteworthy in that
it considers goals, but only uses them indirectly during
construction of standalone concepts. Note that the input
spaces are given to—rather than selected by—Divago. In
Divago, goals do not directly participate in the selection of
elements being projected. If an element a is mapped to b,
one of 4 things can be projected into the blend: a, b, a/b, or
nothing. This leads to a combinatorial explosion of
possible blends regardless of the complexity of finding an
inter-space analogical mapping. To effectively search an
exponentially growing space, Divago utilizes a genetic
algorithm (GA) that stochastically samples the space of
possible blends. From a population of blends, Divago first
selects those with high scores as computed by an
evaluation function, consisting of the weighted sum of the
eight optimality criteria introduced earlier. One of the
criteria, relevance, is interpreted as goal satisfaction. After
evaluation, highly ranked blends are randomly modified to
create the next population. Imitating biological evolution,
this process repeats in the hope of finding a near-optimal
blend after sufficient number of iterations. Note however
that a high score does not guarantee goal satisfaction
because the evaluation function contains several, possibly
competing criteria.
 To elaborate a blend, Divago fires any production rules
whose premises are true to add content into the blend.
Divago also supplements details to the blend based on its
similarity with other frames. For example, if a blend is
similar enough to the bird frame, Divago will grant it the
ability to fly. However, given enough rules and frames,
rule firing and pattern matching can potentially go on
indefinitely, as it does not specify any explicit stopping
criteria for elaboration based on the notions of
meaningfulness or necessity.
 In the light of the above analysis, it is clear that goals
and context can guide a computational conceptual blending
process for the purpose of creating novel, standalone
concepts. However, context and goals must be used
directly to ensure successful blends and to focus search
efficiently. In the next section, we study two creative
systems that utilize contexts and goals to effectively realize
the three procedures in conceptual blending.
Two Case Studies
This section presents two systems that implement blending
theory in a goal and context driven manner. We describe
these systems through the lens of the context-driven
blending theory. The first system builds fictional gadgets
in computer-generated stories. The second system
constructs objects used in pretend play that combine
features of a desired fantasy-world object with a real-world
object at hand. The two systems address the three
aforementioned problems—input space selection, selective
projection, and elaboration—in a neo-Lamarckian manner
by employing constraints introduced by the domain of
application and the specific goal to be achieved. The first
case study focuses on selective projection and elaboration.
The second case study focuses on input space selection.
Generating Gadgets in Fictions
As a vibrant research field, artificial intelligence (AI) story
generation aspires to create intelligent systems that can
create and tell novel stories. Most current approaches to
story generation are restricted to generating stories for
static, hand-authored micro-worlds, manipulating given
characters and objects to produce stories (e.g. Cavazza,
Charles, and Mead 2002; Gervás et al. 2005; Riedl and
Young 2010; Ontañón and Zhu 2010). These systems can
be likened to jigsaw puzzle solvers who play only with
given pieces and never dream of inventing a new piece.
These story generation systems are not able to tell stories
with novel objects or gadgets; they cannot tell Star Wars if
the idea of lightsaber is not supplied ahead of time.
 The aim of the gadget generation algorithm (Li and
Riedl 2011a, 2011b) is to break out of these static world
configurations and create new types of objects previously
unknown to the system. Our approach was initially
presented as a combination of partial-order planning (Weld
1994) and analogical reasoning. Here we point out its
connection to conceptual blending. The algorithm blends
existing concepts to generate novel standalone concepts as
unforeseen gadgets in support of a goal derived from a
story context.
 To generate a gadget, the algorithm reasons about how
the gadget should be used in the context of a story,
including events that happen immediately before and after
its usage. These events are captured as the behavior of the
gadget, represented as a temporally ordered sequence of
actions. Given a goal derived from a story (e.g., a character
must become infected by a flu virus), the algorithm
iteratively constructs the gadget’s behavior by working
backward from the goal using actions and entities from
various input spaces. Goals are first-order logic predicates
such as infected-by(bob, virus). An action is an
operator that requires certain predicates as preconditions
and asserts some predicates as effects. The gadget
generator works with a conventional story generator, which
supplies goals considered appropriate for a gadget to
achieve. The final behavior of the gadget must achieve
these goals.
 Goals are first used to identify the input spaces. The
reference space includes objects in the goal predicates and
relevant concepts. For example, the reference space
implied by the goal infected-by(bob, virus) includes
concepts such as flu viruses, a character named Bob, and
actions such as coughing and curing. In fact, the reference
space exists only conceptually and is not separated from
the rest of the knowledge in the system. It highlights that
knowledge structures closely related to the goal play more
important roles in projection than the rest. The system
International Conference on Computational Creativity 2012 12
creates the second input space by retrieving an object from
its knowledge base of many known objects that achieve
predicates analogous to the specified goals. This object is
called the prototype for the gadget. At this time, the list of
known potential prototypes is small, and we go down the
list from the best guess. In the long term, a robust
mechanism to find the best prototype is needed (e.g.
Wolverton 1994). The second input space, the prototype
space, includes the prototype object, its behavior, as well
as actions used and entities referred to in the behavior. For
example, the behavior of a toy phone object refers to the
entity voice, which becomes part of the space.
 The behavior of the gadget, or the blend space, is built
by projecting actions from the two input spaces selectively.
This projection is driven by goals in a backward-chaining,
iterative process. Following the partial-order planning
representation, actions have causal requirements—
predicates that must be true in the micro-world for the
action to be performed (also called preconditions). When
an action is brought in the blend space to solve one goal,
its own causal requirements are added to the set of goals.
Actions continue to be projected to satisfy goals until all
goals are satisfied or determined to be fundamental
properties of the gadget itself. Note that when there are
multiple possible ways to achieve a goal, the algorithm
tries the best first and backtracks when mistakes are made.
The selective projection process is thus completely goaldriven.
Due to space constraints, we refer interested reader
to (Li and Riedl 2011b) for details of the algorithm.
 There are several methods to project actions from the
input spaces into the blend space. First, the algorithm can
project an action from the prototype space without any
changes. Second, an action from either space can be
projected with arguments from both spaces, constituting a
form of blending. Figure 3(a) shows the action CoughInto
being projected with arguments from both input
spaces. This action is mapped with the action Speak-Into
in the prototype space because of identified analogical
similarities. The analogical reasoning engine Sapper
(Veale and O'Donoghue 2000) is used to establish
analogical mapping across input spaces. Third, a special
case of blending occurs when an action from the prototype
space is combined with illegal arguments from the
reference space to create a new action previously not
allowed. As illustrated in Figure 3(b), the Transmit action
of a toy phone is now allowed to transmit virus instead of
voice, which otherwise would not have been a legal
assignment of parameters. The algorithm ignores the rules
of the micro-world to achieve goals of an imaginary gadget
phone that can transmit flu virus from one person to
another. This allows a generated gadget to achieve the
impossible, producing an object not conceived before.
 Figure 4 shows the behavior of a gadget phone
generated by the algorithm, which one can use to give her
flu to someone else and free herself from it. The goal
predicates it achieves are infected-by (bob, virus) and
not(infected-by (alice, virus)). Each box
represents one action. Solid arrows denote temporal
precedence and dotted arrows denote closure actions. This
gadget appeared in the Doraemon manga. Li and Riedl
(2011a) also describes an example not from the manga.
 Elaboration in the blend space is simulated with the use
of closure actions. Closure actions are not necessary for the
gadget’s goal, but restore some goal-irrelevant aspects of
the story world to an ordinary state. For example, in Figure
4, the actions where people put down the phone after use
(the Let-Gos) are closure actions. Closure actions are
manually labeled in prototype behaviors and projected into
the blend space. Their use is motivated by the desire to
restore an ordinary state after using the gadget, which
creates a sense of denouement in the story. Although the
elaboration is not directly driven by goals, it is motivated
by the domain of storytelling.
 Finally, the gadget is verified by incorporating the
gadget behaviors into the story from which the goals were
originally derived. If any necessary conditions of the
gadget’s behavior cannot be achieved in the story, the
gadget does not work and has to be modified further by
going through additional rounds of blending. When this
happens, a second prototype is retrieved as yet another
input space and blended with the current gadget to make it
even more powerful. The algorithm cannot create two
gadgets simultaneously.
Figure 4. The behavior of the flu-transmitting gadget phone
Figure 3. Two projections involved in the generation of the flutransmitting
gadget phone.
International Conference on Computational Creativity 2012 13
 By using means-ends search guided by goals, the search
space of possible actions to be added to the behavior of the
gadget is pruned, resulting in improved average-case
efficiency of the algorithm. Each iteration of the algorithm
tries to satisfy a goal in the blend space, and will thus only
consider actions that can achieve that goal. While the total
number of actions in both input spaces may be large,
usually only a small fraction of them can be projected to
achieve any given goal. In contrast, a naïve selective
projection algorithm will attempt to project all actions in
any input space using all possible projection methods.
Although the goal-driven best-first search in the worst case
has to consider all possibilities in the search space, in an
average case it only considers a small portion of the total
possibilities (Weld 1994). A goal-driven blending
algorithm also has the added benefit of ensuring that any
result of the algorithm is guaranteed to meet all of the
acceptability requirements.
 In summary, the gadget generation system implements a
goal-driven model of blending, including relatively
efficient selective projection (due to pruning of the search
space) and elaboration procedures. These procedures are
driven by the gadget’s goal, the particular story that serves
as the context, and the general domain of storytelling.
However, this system has not fully investigated the
selection of input spaces, which will be illustrated in the
next case study.
A Virtual Character for Pretend Play
Our second case study illustrates the use of goals in
blending with a specific focus on the selection of
appropriate input spaces. In this case, a real-world object is
selected to represent an object from a fantasy world, as
required in children’s pretend play. A goal specifies means
to appropriately prune potential input spaces and select one
option based on contextual constraints of similarity. Below
we describe how our pretend play system can be viewed
through the lens of conceptual blending; full details on the
system are presented in (Zook, Riedl, and Magerko 2011).
 In pretend play, children construct and enact story
scripts and roles with real-life objects (Nourot 1998).
Examples of pretend play include lightsaber duels with
cardboard tubes, holding pretend tea parties with stuffed
animal guests and acting as a group of pirates sailing on a
couch. When enacting these scripts, pretenders have goals
of using particular objects from a fictional world, but are
limited to using the real-world objects that are ready at
hand. Pretend players imaginatively overlay the fictional
object onto the real-world object to create a blend, i.e. a
pretend object existing in both the fictional world and the
real world. Pretend play research has found children
project traits of the fictional object on the real object. As an
example, children engaged in a lightsaber duel with
cardboard tubes may make buzzing noises when they
swing the tube. In general, the pretending process involves
identifying real-world objects as stand-ins for the fictional
object, and selectively projecting traits of the fictional
object onto the real object. The objects are input spaces to
the blend. The construction process must account for how
input spaces relate to a larger context of a target pretend
play activity, pruning the options considered.
 Building computational systems that can engage in
pretend requires the capacity to construct the objects used
in these scripts (Zook, Riedl, and Magerko 2011). To
formalize the problem, the play activity (lightsaber duel)
provides a structuring situation such that a pretender—
human or agent—selects a real world object (cardboard
tube) as a presentation space for a given reference space of
a fictional object (lightsaber). That is, the goal is to find a
presentation input space that most closely matches the
reference input space. Once the input spaces are selected,
the blending process takes the most relevant aspects of the
fictional object for the activity (buzzing), which are
imposed onto the real object in the blend space for use in
play (swinging a cardboard tube while buzzing). This
process starts with a situation in the fictional world and a
specific fictional object (the lightsaber I am using in the
duel), and seeks a presentation object in the real world to
effectively manifest the fictional object.
 To reason about numerous objects in the fictional world
and the real world, we need a computational representation
of objects and their attributes. Lakoff and Johnson (1980)
proposed that the salient perceptual, motor-activity, and
purposive features of objects affect how humans interact
with them. We model objects in both the fictional and real
domains using selected attributes in these categories.
Following prototype theory (Rosch 1978), these attributes
are assigned fuzzy values to represent a real-valued ([0, 1])
range of degree of membership (DOM). As an example, a
lightsaber may have a 0.8 DOM value for the perceptual
feature of being blue (very blue, except for the handle), 0.9
DOM value for the motor-activity feature of ease of
handling (very easy to hold and swing), and 0.1 DOM
value for the purpose of supporting weights (unsuitable for
propping up heavy objects).
Iconic attributes are salient attributes of an object that
distinguish it from similar objects within the same
category. These attributes help to resolve the potential
ambiguity of which fictional object is being represented by
a given real world object. For example, if a pretender grabs
an object and begins making buzzing noises, it may be
unclear if they are signaling that they are holding a buzzing
lightsaber or shooting a laser pistol. An iconic posture of
handling, however, makes this difference clear. Iconic
attributes help participants in pretend play interpret other
players’ behaviors and intentions.
 The computational play system algorithm has three steps
for context and goal driven blending: (1) select a realworld
object based on the pretending goal and context;
(2) select the set of fictional object attributes to project;
and (3) project these attribute values into the blend. The
first step is the selection of the input space—the real-world
object—to be blended with the fictional object.
 Selecting an input space uses the pretending goal to first
prune impossible input spaces and then search for the
optimal input space among those that remain. Conceptually
International Conference on Computational Creativity 2012 14
this process is similar to the surface-level filtering process
used in the MAC/FAC system (Forbus, Gentner and Law
1995) with the modification of using fuzzy attributes rather
than predicates. The filtering quickly removes from
consideration all real-world objects that differ too
significantly from the desired fictional object on a single
attribute. The goal specifies attributes that are important to
the pretend play object and the extent to which they must
be preserved. Thus, when seeking a lightsaber, all real
objects that are too difficult to handle would be ignored, as
they cannot serve as useful lightsabers during the play.
Computationally, all real objects are compared to the
desired fictional object on the set of relevant attributes, and
those that differ by a specified threshold are pruned. For
example, when considering ease of handling, a cardboard
tube would be kept as a candidate real object for a
lightsaber, while a wooden log would be discarded.
 After filtering, the remaining real-world objects are
searched for the single real-world object with minimum
difference from the desired fictional object.
Computationally, the pretend play system exhaustively
searches all available real-world objects and calculates the
Euclidean distances between the attributes of each
candidate real object and the fictional object. The realworld
object with the minimal distance from the fictional
object is then selected. In this domain, pruning appears to
sufficiently constrain the set of potential input spaces to
enable subsequent exhaustive search, although alternative
search techniques are likely applicable.
 Once the input spaces are selected, the second step is to
bridge the remaining distance between the real-world
object and the fictional object by determining the set of
iconic attributes that capture characteristic attributes of the
fictional object. These will then be mapped back to the
real-world object so that the agent can play with the realworld
object as a placeholder for the fictional object.
Iconic attributes of an object are those attributes that are
most different from other objects under consideration; they
capture which features are relevant to the pretend goal. The
level of iconicity of an attribute for the desired pretend
object is calculated as the sum of Euclidean distances
between that attribute and the same attribute of all other
objects in the fictional world. Iconicity values are
normalized within categories of objects and an attribute is
considered iconic for an object if it falls in the proximity of
the maximum value.
 In the third stage, blending occurs by projecting iconic
values of the desired fictional object onto the selected real
object. By default, the blend space contains all attributes of
the real object. All iconic attributes of the pretend object
are projected into the real object, replacing the original
values. This process captures the notion that most action
and reasoning should treat the real world as the base with
the pretend domain layered onto this base.
 In the pretend play algorithm, context and goals are
utilized to filter the set of possible input spaces to only
those that are most crucial for the use of a real-world
object for pretend play. By pruning the set of possible
objects according to their relevance to a goal, the process
avoids the naïve consideration of all possible combinations
of spaces to use for blending. Selective projection of
attributes is achieved by searching for the most iconic
attributes of the presentation input space (the fictional
object) to be blended with attributes of the reference input
space (the real-world object). While this search is
performed in a brute-force manner, the number of
attributes that cross the acceptability threshold for iconicity
are relatively limited.
Discussion and Conclusions
As a powerful mechanism for creativity, conceptual
blending is capable of synthesizing known concepts into
new concepts. Much existing theoretical work focuses on
the blending phenomenon and identifying the input spaces
and the blend without a mechanism for blending. This
paper presents our first efforts at building a complete
computational outline for conceptual blending systems. We
identify three major procedures in conceptual blending: (1)
the selection of input spaces, (2) the mechanism for
selective projection of input space attributes into the blend
space, and (3) the sufficiency condition for pattern
completion and elaboration. These components play vital
roles in conceptual blending and have significant
implications for the efficiency of computation. In our
analysis of computational implementations of blending
theory, we found few systems fully account for all three
processes.
 Brandt and Brandt (2002) argued that the construction of
semiotic expressions as blends are cued by communication
contexts and guided by the specific communicative goal.
We argue that context and goals can provide the basis for
rigorous and efficient computational algorithms for the
three main processes described above. We present two
computational systems that utilize goals and context to
guide generation of standalone conceptual blends. The
gadget generation algorithm mainly demonstrates
procedures (2) and (3) by utilizing goals to select concepts
from the input spaces and elaborating them. The pretend
play work likewise uses context to determine which input
spaces to select and which concepts from the input spaces
to project into the blend, illustrating procedures (1) and (2).
 These two case studies suggest the three main
procedures can be implemented efficiently by employing
constraints introduced by their respective domain of
application, the contexts of the solutions, and the specific
goals the solutions must achieve. Our analysis shows that
goals can be used to prune the search space and improve
average-case performance. Although our implementations
are deterministic, we believe determinism and goals are not
a bundled package. A goal-driven procedure may not be
completely deterministic or even optimal. Future work is
needed to reduce the effort required to author the
knowledge representations used by our systems (e.g. with
crowdsourcing (Li et al. 2012)).
 Boden (2004) raised the questions of whether computers
can appear to be creative, and whether computational
International Conference on Computational Creativity 2012 15
systems can help us understand creativity. By
implementing theories of creativity, we are forced to
consider procedural details which theories sometimes do
not cover. We believe a computational approach can help
expose underspecified components or flaws in existing
theories, hint at their solution, or even lead to their remedy.
A computational approach to creativity will strengthen our
confidence in answering yes to both of Boden’s questions.
Acknowledgement
This work was supported by the National Science
Foundation under Grant No. IIS-1002748. We thank Per
Aage Brandt and the anonymous reviewers for valuable
inputs.
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