We now turn to stating some intuitive principles of information. We use such terms as `fact' and `situation' in their ordinary senses; in the next section we give explications of them within situation theory.
We take the second sort of information report, exemplified by (3), as canonical. Where X is a noun phrase designating the carrier of the information, `X indicates that such and such' is elliptical for `X's being so and so indicates that such and such'. For example, we bring Jackie, who has been limping badly, to the vet, who takes an x-ray of Jackie's left hind leg, the one she's been favoring. The x-ray is developed. At this point the vet might say something fully explicit like (3).
So our first principles are as follows:
(A) Facts carry information.
(B) The informational content of a fact is a true proposition.
What underlies the phenomenon of information is the fact that reality is lawlike; that what is going in one part of reality is related to what is going on in some other part of reality, by laws, nomic regularities, or as we shall say, constraints. Our point of view may be taken as a generalization of Hume's. He took constant conjunctions to be contingent matters of fact, that one type of event was constantly conjoined with another. We take constraints to be contingent matters of fact, that one type of situation involves another. Involving implies constant conjunction: if one type of situation involves another, then if there is a situation of the first type, there is one of the second type. But we leave open the question of whether constant conjunction implies involvement.
In a world knitted together by constraints--whether these be constant conjunctions or some more metaphysically potent connections--situations carry information. The fact that there is a situation of one type, carries the information that there are situations of the types that one involves. If it is a constraint that objects left unsupported near the surface of the earth fall, then the fact that a certain apple near the surface of the earth is left unsupported, carries the information that it will fall.
This conception licenses the notion of the information carried by a fact relative to a constraint. It is this relative notion of informational content that we think is implicit in our actual thinking about information and important for theoretical purposes. From it one might derive an absolute notion of the information carried by a situation, as that information carried by the situation relative to some constraint or other. We do not think this is a useful notion.
(C) The information a fact carries is relative to a constraint.
Hume saw constant conjunction as supplying the world with enough structure so that events contained information, which experience enabled us to recognize. But this structure did not require that there be any intrinsic connections between events; no event contained information in virtue of its intrinsic properties. If the event were embedded in a different sort of world, where different constraints held sway, it would carry quite different information than it actually does.
(D) The information a fact carries is not an intrinsic property of it.
The informational content of a fact can involve objects quite remote from those involved in the fact. Jackie is not a part or aspect of the x-ray mentioned in (3), nor is she a constituent of the fact that the x-ray has such and such a pattern, but something remote from it. The x-ray is not broken, and does not have bones. Information typically involves a fact indicating something about the way things are elsewhere and elsewhen, and this is what makes information useful and interesting.
(E) The informational content of a fact can concern remote things and situations.
This conception of information can explain how an x-ray could carry the information that some dog had been x-rayed and had a broken leg. But it is not clear how it can account for the specific information the x-ray carries about Jackie that is reported in (3). As we noted above, Jackie is not a part of the x-ray, and it does not seem that her having a broken leg could be constantly conjoined with x-rays exhibiting the pattern that the vet recognizes. So how can the informational content of the x-ray have her as a constituent?
We shall call the sort of information reported, e.g., in (3) incremental information. The conception is most easily grasped if we think of what the x-ray indicates to the vet. If she does not know which dog the x-ray is of, it simply indicates that some dog has been x-rayed that has a broken leg. We call this the pure information. But if she knows that Jackie was x-rayed, then the pattern on the x-ray indicates to her the additional or incremental information that Jackie has a broken leg. This is the incremental information carried by the x-ray, given the fact that the x-ray is of Jackie. The fact that is given connects the indicating situation and the specific objects the information is about, so we shall call it the connecting fact. We must be careful that this example does not mislead as to our intentions, however. Incremental information is important in understanding the use humans make of information, but humans and mental states need not be brought into its analysis. Incremental information about specific objects is an objective feature of the world that is there for us to use.
(F) Informational content can be specific; the propositions that are informational contents can be about objects that are not part of the indicating fact.(G) Indicating facts contain such information only relative to connecting facts; the information is incremental, given those facts.
If we put the x-ray in a drawer for a day or a month, it will still indicate that Jackie had a broken leg. After a month, of course, it will not indicate that Jackie has a broken leg then, for the leg might have mended. It will still indicate that Jackie had a broken leg at the time the x-ray was taken. This illustrates two important points about information.
First, different facts can carry the same information. Suppose that t is the time the x-ray was taken, and t' is a month later. The fact that the x-ray exhibits a certain pattern at t and the fact that it exhibits that pattern at t' are different, yet they carry the same information. And of course many facts, exhibiting more radical differences from the original state of the x-ray, could carry the same information about Jackie's leg--the way she limped, the vet's remarks after feeling the leg, the notations in Jackie's file years later.
In the case of the stored x-ray, though, the later fact contains the information that Jackie had a broken leg because the earlier one did. And this is the second point illustrated by the example. For this is a (very) simple case of the storage of information. Note that what goes on in this case is that the carrier of the information is itself stored, in the drawer say, whence it and the information it carries can be retrieved. This storage system works so long as the manner in which the indicating object is stored preserves the indicating property. The world is to be arranged in such a way that the carrier has the indicating property over a usefully long stretch of time. No storage system works forever.
Now imagine that a xerox is made of the x-ray and the copy sent to a vet in another city. It, too, will indicate that Jackie has a broken leg. That is, it's having such and such a pattern will indicate that Jackie has a broken leg. This is a simple case of the flow of information. Notice here that it's crucial that the copy be a copy, that is, that it be related in a certain way to the original carrier and its indicating property. But things can be otherwise. In some cases storage and transmission of information involves varying both the carrier and the indicating properties. Thus, the x-ray's indicating pattern could be digitized and sound waves produced by scanning the binary array. (This process could even be reversible, up to the stipulated quantization.)
Later we look closely at a more complex case of the flow of
information due to Dretske. An announcer
speaks into a microphone, the microphone is connected through a
transmitter to a transmitting antenna. The modulation of the
electromagnetic signal transmitted by the antenna contains information
about what the announcer says, because it contains information about
the way the microphone diaphragm vibrates, and that contains
information about the voice.
(H) Many different facts, involving variations in objects, properties, relations and spatiotemporal locations, can indicate one and the same informational content--relative to the same or different constraints.(I) Information can be stored and transmitted in a variety of forms.
The x-ray's being such and such at t carries the information that Jackie's left hind leg is broken; but what good does this do the x-ray? None. What good does it do Jackie? Perhaps, a lot. If, that is, the vet has the information that Jackie has a broken leg, the chances of her doing Jackie some good increase dramatically. It has often been noted that books contain a lot of information, too, yet that fact seems not to be of any use to books. There is a distinction between carrying or containing information and having information. We shall suggest that an agent or device has the information that P just in case it is in a state that both carries the information that P, and controls the behavior of the device in a way appropriate to the truth of P.
(J) Having information is good; creatures whose behavior is guided or controlled by information (by their information carrying states) are more likely to succeed than those which are not so guided.
There is a certain tension between (J) and the rest of our principles. They all emphasize that the information carried by an agent or device being in a certain state is a contingent, extrinsic fact about that agent or device. Given different constraints or specific facts, the information carried by the agent or device being in the state would be quite different. Yet the effect that being in the state has on the device, cannot depend on these remote contingencies. How then can such states control behavior in ways that are appropriate to this information? We shall see, however, that when we think through our principles in a careful way, this tension is only apparent.