<s>
In	O
a	O
formal	O
system	O
of	O
logic	O
used	O
for	O
knowledge	O
representation	O
,	O
the	O
open-world	B-Application
assumption	I-Application
is	O
the	O
assumption	O
that	O
the	O
truth	O
value	O
of	O
a	O
statement	O
may	O
be	O
true	O
irrespective	O
of	O
whether	O
or	O
not	O
it	O
is	O
known	O
to	O
be	O
true	O
.	O
</s>
<s>
It	O
is	O
the	O
opposite	O
of	O
the	O
closed-world	B-Application
assumption	I-Application
,	O
which	O
holds	O
that	O
any	O
statement	O
that	O
is	O
true	O
is	O
also	O
known	O
to	O
be	O
true	O
.	O
</s>
<s>
An	O
open-world	B-Application
assumption	I-Application
was	O
first	O
developed	O
by	O
Ancient	O
Greek	O
philosophers	O
as	O
a	O
means	O
to	O
explain	O
varying	O
degrees	O
of	O
validity	O
amongst	O
mathematical	O
and	O
philosophical	O
concepts	O
proposed	O
at	O
the	O
time	O
of	O
inception	O
.	O
</s>
<s>
The	O
open-world	B-Application
assumption	I-Application
(	O
OWA	O
)	O
codifies	O
the	O
informal	O
notion	O
that	O
in	O
general	O
no	O
single	O
agent	O
or	O
observer	O
has	O
complete	O
knowledge	O
,	O
and	O
therefore	O
cannot	O
make	O
the	O
closed-world	B-Application
assumption	I-Application
.	O
</s>
<s>
In	O
contrast	O
,	O
the	O
closed	B-Application
world	I-Application
assumption	I-Application
allows	O
an	O
agent	O
to	O
infer	O
from	O
the	O
lack	O
of	O
knowledge	O
of	O
a	O
statement	O
being	O
true	O
that	O
the	O
statement	O
being	O
false	O
.	O
</s>
<s>
Heuristically	O
,	O
the	O
open-world	B-Application
assumption	I-Application
applies	O
when	O
we	O
represent	O
knowledge	O
within	O
a	O
system	O
as	O
we	O
discover	O
it	O
,	O
and	O
where	O
we	O
cannot	O
guarantee	O
that	O
we	O
have	O
discovered	O
or	O
will	O
discover	O
complete	O
information	O
.	O
</s>
<s>
Semantic	O
Web	O
languages	O
such	O
as	O
OWL	B-Language
make	O
the	O
open-world	B-Application
assumption	I-Application
.	O
</s>
<s>
Many	O
procedural	B-Application
programming	I-Application
languages	I-Application
and	O
databases	O
make	O
the	O
closed-world	B-Application
assumption	I-Application
.	O
</s>
<s>
The	O
closed-world	B-Application
assumption	I-Application
typically	O
applies	O
when	O
a	O
system	O
has	O
complete	O
control	O
over	O
information	O
;	O
this	O
is	O
the	O
case	O
with	O
many	O
database	O
applications	O
where	O
the	O
database	B-General_Concept
transaction	I-General_Concept
system	O
acts	O
as	O
a	O
central	O
broker	O
and	O
arbiter	O
of	O
concurrent	O
requests	O
by	O
multiple	O
independent	O
clients	O
(	O
e.g.	O
,	O
airline	O
booking	O
agents	O
)	O
.	O
</s>
<s>
The	O
open-world	B-Application
assumption	I-Application
is	O
closely	O
related	O
to	O
the	O
monotonic	O
nature	O
of	O
first-order	O
logic	O
:	O
adding	O
new	O
information	O
never	O
falsifies	O
a	O
previous	O
conclusion	O
.	O
</s>
<s>
The	O
language	O
of	O
logic	O
programs	O
with	O
strong	O
negation	O
allows	O
us	O
to	O
postulate	O
the	O
closed-world	B-Application
assumption	I-Application
for	O
some	O
statements	O
and	O
leave	O
the	O
other	O
statements	O
in	O
the	O
realm	O
of	O
the	O
open-world	B-Application
assumption	I-Application
.	O
</s>
<s>
Under	O
the	O
PCWA	O
,	O
the	O
knowledge	O
base	O
is	O
generally	O
treated	O
under	O
open-world	B-Application
semantics	I-Application
,	O
yet	O
it	O
is	O
possible	O
to	O
assert	O
parts	O
that	O
should	O
be	O
treated	O
under	O
closed-world	O
semantics	O
,	O
via	O
completeness	O
assertions	O
.	O
</s>
