In artificial intelligence, a fluent is a condition that can change over time.
For example, the condition "the box is on the table", if it can change over time, cannot be represented by
This representation of fluents is modified in the situation calculus 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.
whose value is the object the box is standing on at time
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
From a historical point of view, fluents were introduced in the context of qualitative reasoning.
The idea is to describe a process model not with mathematical equations but with natural language.
That means an action is not only determined by its trajectory, but with a symbolic model, very similar to a text adventure.
[1] The fluent realizes the common sense grounding between the robot's motion and the task description in natural language.
[2] From a technical perspective, a fluent is equal to a parameter that is parsed by the naive physics engine.
The parser converts between natural language fluents and numerical values measured by sensors.
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