Fluent (artificial intelligence)

In <a href="/facts/Artificial_intelligence/lJGauQwX">artificial intelligence</a>, a fluent is a condition that can change over time. In <a href="/facts/Logic/8JnpJLtt">logical</a> approaches to reasoning about actions, fluents can be represented in <a href="/facts/First-order_logic/lcISqWIg">first-order logic</a> by <a href="/facts/Predicate_(logic)/5WazAvKF">predicates</a> having an argument that depends on time. For example, the condition "the box is on the table", if it can change over time, cannot be represented by 
 
 
 
 
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 {\displaystyle \mathrm {On} (\mathrm {box} ,\mathrm {table} )}
 
; a third argument is necessary to the predicate 
 
 
 
 
 O
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 {\displaystyle \mathrm {On} }
 
 to specify the time: 
 
 
 
 
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 {\displaystyle \mathrm {On} (\mathrm {box} ,\mathrm {table} ,t)}
 
 means that the box is on the table at time 
 
 
 
 t
 
 
 {\displaystyle t}
 
. This representation of fluents is modified in the <a href="/facts/Situation_calculus/dGcIavPa">situation calculus</a> by using the sequence of the past actions in place of the current time.
A fluent can also be represented by a function, dropping the time argument. For example, that the box is on the table can be represented by 
 
 
 
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 {\displaystyle on(box,table)}
 
, where 
 
 
 
 o
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 {\displaystyle on}
 
 is a function and not a predicate. In first-order logic, converting predicates to functions is called <a href="/facts/Reification_(knowledge_representation)/4udgqY99">reification</a>; for this reason, fluents represented by functions are said to be reified. When using reified fluents, a separate predicate is necessary to tell when a fluent is actually true or not. For example, 
 
 
 
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 ,
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 {\displaystyle HoldsAt(on(box,table),t)}
 
 means that the box is actually on the table at time 
 
 
 
 t
 
 
 {\displaystyle t}
 
, where the predicate 
 
 
 
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 {\displaystyle HoldsAt}
 
 is the one that tells when fluents are true. This representation of fluents is used in the <a href="/facts/Event_calculus/82A2yW5H">event calculus</a>, in the <a href="/facts/Fluent_calculus/Ieuy6qR4">fluent calculus</a>, and in the features and fluents logics.
Some fluents can be represented as functions in a different way. For example, the position of a box can be represented by a function 
 
 
 
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 {\displaystyle on(box,t)}
 
 whose value is the object the box is standing on at time 
 
 
 
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 {\displaystyle t}
 
. Conditions that can be represented in this way are called functional fluents. Statements about the values of such functions can be given in first-order logic with equality using literals such as 
 
 
 
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 {\displaystyle on(box,t)=table}
 
. Some fluents are represented this way in the <a href="/facts/Situation_calculus/dGcIavPa">situation calculus</a>.

Fluent (artificial intelligence) open-in-new

Fluent (artificial intelligence)