The majority of the following discussion comes from Davis et al., “What is a knowledge representation?”
A knowledge representation is:
a surrogate for real-world entities
an ontological commitment about what kinds of entities are relevant
a fragmentary theory of intelligent reasoning, expressed in terms of three components: (1) the KR’s idea of what makes intelligent reasoning, (2) the inferences that are possible in the KR, and (3) the inferences that it “recommends”
a medium for pragmatically efficient computation; the KR offers guidance for organizing information to facilitate making the recommended inferences
a medium for human expression
KR is a surrogate
Reasoning about the world is an internal process. If an agent wishes to plan how to assemble a bike then the agent must “think” about wheels, chains, handle bars, and so on. These entities must exist in the agent’s “mind” if it is to think about them. We expect the agent to be able to plan how to assemble a bicycle without needing to physically manipulate bicycle parts.
Thus, the knowledge representation provides a surrogate for real-world entities. If the KR is useful, the actions that can be performed internally (as the agent is “thinking”) will more-or-less match what the corresponding real-world actions do to real-world entities. For example, the agent’s understanding of what “lock the wheel into the frame” does to a wheel and a frame should match the real world (the wheel is now stuck to the frame, and if the frame is lifted, the wheel comes with it, etc.).
For any entity (surrogate) in the knowledge representation, we have to be sure we have a good argument for:
“correspondence”: what does this entity in the KR represent in reality?
“fidelity”: how well does it match what it is supposed to represent?
All representations are imperfect; some detail is always left out (for pragmatic concerns if not oversight). Things can go wrong. Locking the wheel into the frame may not work on some occasion (say, the frame is bent). We will never be able to represent all possibilities.
KR is an ontological commitment
All representations are imperfect, and details must inevitably be left out; furthermore we typically do not attempt to represent every object and fact in the known universe. Thus, whenever we use a knowledge representation, we are making a (possibly implicit) commitment to the existence of certain entities and not others. If we want to build an agent that can assemble and fix bicycles, that agent presumably has no use for facts about the Napoleonic Wars. So to the bicycle-fixing agent, those historical facts simply do not exist. It has no way of thinking about them.
Davis, et al. describe this feature of KRs as the “focusing” or “glasses” effect, because the agent is effectively wearing glasses that limit its focus.
The focusing effect is an essential part of what a representation offers because the complexity of the natural world is overwhelming. We (and our reasoning machines) need guidance in deciding what in the world to attend to and what to ignore. The glasses supplied by a representation can provide this guidance: In telling us what and how to see, they allow us to cope with what would otherwise be untenable complexity and detail. (op cit.)
Figuring out what ontological commitments to make, that is, what is relevant to the agent, is a big problem. Also a problem is giving the agent an understanding of what is not represented. Assembling a bicycle in zero gravity is likely much more difficult; so can the agent just assume it is operating in Earth-like gravity? Why can it make that assumption? Where is that assumption in the agent’s programming and knowledge?
These problems are elucidated by John McCarthy, by way of an example:
Three missionaries and three cannibals come to a river. A rowboat that seats two is available. If the cannibals ever outnumber the missionaries on either bank of the river, the missionaries will be eaten. How shall they cross the river?
Obviously the puzzler is expected to devise a strategy of rowing the boat back and forth that gets them all across and avoids disaster […]
Imagine giving someone the problem, and after he puzzles for a while, he suggests going upstream half a mile and crossing on a bridge. ‘What bridge?’ you say. ‘No bridge is mentioned in the statement of the problem.’ And this dunce replies, ‘Well, they don’t say there isn’t a bridge.’ You look at the English and even at the translation of the English into first order logic, and you must admit that ‘they don’t say’ there is no bridge. So you modify the problem to exclude bridges and pose it again, and the dunce proposes a helicopter, and after you exclude that, he proposes a winged horse or that the other hang onto the outside of the boat while two row.
You now see that while a dunce, he is an inventive dunce. Despairing of getting him to accept the problem in the proper puzzler’s spirit, you tell him the solution. To your further annoyance, he attacks your solution on the grounds that the boat might have a leak or lack oars. After you rectify that omission from the statement of the problem, he suggests that a sea monster may swim up the river and may swallow the boat. Again you are frustrated, and you look for a mode of reasoning that will settle his hash once and for all. — J. McCarthy, “Circumscription—A form of non-monotonic reasoning,” 1980
KR is a fragmentary theory of intelligent reasoning
The reason we build and employ knowledge representations is to facilitate inferencing or reasoning that’s intelligent. The KR embeds a theory of intelligent reasoning. This theory is fragmentary because we have (so far) been unable to build a KR, with an accompanying inferencing procedure, that captures all intelligent reasoning. One might be able to argue that, in principle, building such a complete theory is impossible.
There are three fundamental questions related to a KR’s embedded theory of intelligent reasoning:
What is intelligent reasoning?
The field of AI has discovered/invented many varieties of intelligent reasoning. We have deduction (which is what we’ll study), induction, abduction, reasoning by analogy, probabilistic reasoning, case-based reasoning, etc. Each of these are closely related to certain knowledge representation data structures.
What can we infer from what we know?
Deduction famously cannot yield any new facts; a deductive conclusion does not state anything more than what was already present in the premises. However, induction and abduction, for example, are “ampliative” forms of reasoning because they offer new facts (which may be wrong; deduction, on the other hand, is never wrong).
If we want ampliative inference, we have to keep this in mind as we build the knowledge representation. In this case, deduction cannot be our only inference tool.
What should we infer from what we know?
Often the set of inferences available to use is enormous. Which just a handful of premises and the method of deduction, we can often deduce many irrelevant facts. How does the agent guide the inference process so that it can meet certain goals?
KR is a medium for pragmatically efficient computation
AI systems must eventually do something; they must compute. So, we need our knowledge representation to be computationally pragmatic. It’s easy to think up knowledge representations that are simple, beautiful, precise, and… unwieldy. The tricks for making a KR useful but also pragmatic can be quite sophisticated. We’ll avoid dealing with this issue. However, when we look at Prolog, it is important for us to keep in mind that Prolog is useful precisely because it limits the expressiveness of the KR in order to keep the inferencing computationally efficient.
KR is a medium for human expression
If the representation makes things possible but not easy, then as real users we might never know whether we misunderstood the representation and just do not know how to use it or whether it truly cannot express some things that we would like to say. A representation is the language in which we communicate; hence, we must be able to speak it without heroic effort.
Making it happen
When we actually create systems that solve AI problems, we need to decide on one or more knowledge representations. There are two general categories of KRs: procedural and declarative.
This is the form of knowledge with which you are probably most familiar. Procedural knowledge is, essentially, a program. If you want to give your robot knowledge about how to navigate a maze, you write code to navigate a maze. This code will probably first find the locations of nearby walls and exits, and make a choice about where to move based on criteria embedded in the algorithm.
In another case, an industrial robot may be programmed to perform a certain sequence of actions (maybe with a few variations depending on context), such as assembling part of a car. The “knowledge” is embedded in the procedure (the algorithm).
Compare this description of procedural knowledge with the way cognitive psychologists describe procedural knowledge: they say that procedural knowledge includes recognition of a face or reacting to a joke. For them, it is “tacit” knowledge because it cannot be described by the person who has it; exactly what knowledge is brought to bear is not conscious to the agent. This is similar to our description because knowledge that is embedded in an algorithm likewise is very difficult to extract and explicate outside of its active use.
Note that our prior work with the 8-puzzle used procedural knowledge: the board states were generated based on an algorithm; there was no separate “description of a board” that we consulted.
Knowledge that is explicitly represented separately of its use is declarative knowledge. “I declare that such-and-such is the case!” It is knowledge that is true independently of how it happens to be used in some context.
Procedural knowledge is generally specific to the algorithm in which it is embedded. Thus, there has been less of a drive to create “universal” ways of representing procedural knowledge (well, perhaps we can think of programming languages as universal representations of procedural knowledge).
On the other hand, declarative knowledge, being essentially universal in nature (not tied to a particular active context like procedural knowledge is), has experienced centuries of formalization. If we want to say something universal, perhaps we should have a universal way of saying it!
We’ll look at four ways of “saying” declarative knowledge, each more sophisticated than the last. We’ll explicitly note that these forms of logic have nothing to do with programming (nothing to do with procedures). However, in order to actually build software with declarative knowledge, we’ll next look at a programming language that sort-of approximates a certain logical formalism.