Dear Pat, March 10,1997 I apologize for not answering more directly your central concern about definitions and entailment: I shall rectify that omission in this letter. But first I respond to some related points raised in your previous letter to me. About Aaron's "deductive reduction", I had also originally believed that it was equivalent to entailment, but expressed from the other end, i.e., from top down rather than bottom up. But then I thought I saw that Aaron meant something different here, namely the fact that e.g. a computer program does not reveal how it is to be implemented at the physical level, nor do the rules of chess say anything about their physical implementation. So these "architectures" are somehow floating in another realm of reality: they are not bound into any particular implementation. But the "entailment" that I am talking is another sort of connection: it IS "deductive implication" , within some logical framework that must of course be specified. In the present context this logical framework is classical physical theory, which is often assumed to provide a description of the "physical" aspects of nature that is sufficiently accurate for the needs of a science of consciousness. Entailment refers in this context to deductive implications *from* the facts about nature that are represented in the classical physical theoretical conception of the world *to* facts about nature that are represented in some other language, or at some other level of description. About my description of what I believe to be the orthodox classical viewpoint you exclaim: "This is physics?? If so then physics had better get its act together. Its not neuroscience or psychology, both of which assume that experiences have causal effects on the physical world". I agree completely that physics had better get its act together, precisely because the orthodox classical conception does not allow for causally efficacious consciousness. That is exactly my point! I now have your reply to Gregg . At the end of you make one point very clear: "Of course the physical facts do not entail the phenomenal facts without additional bridging laws. My point is only that it applies just as forcibly to *any* facts that are not couched in the particle-field language of physics." About these additional bridging laws, which we agree are needed, I have never said that they are "invalid", as you claimed. I have only repeatedly stressed that the principles of classical physical theory---and facts specified by the dispositions of the particles and fields that classical theory says exists---do not entail the existence of any phenomenal realities associated with the body/brains of normally behaving human beings, although this classical description certainly does fix/specify/entail their (overt) behaviours. I guess we now agree on this main point! But this point immediately entails that zombies are logically possible within the framework of classical physical theory: the fact that each normal human body/brain has an associated realm of experience is not entailed by the classical physics theoretical description and principles. And any experience that is present is epiphenomal: the behaviour and physical functioning of the body/brain would, according to the classical conceptualization, be just the same whether that experience occurred or not! I regard these features of the classical theory as serious defects, and have been urging a search for a more appropriate physical theory; one in which experiences are both necessarily present and causally efficacious in normally functioning human body/brains. [I do not go here into the great merit in this context of the quantum mechanical conception of nature.But the main point is that in QM the connection between the geometrical and experiential aspects of nature lies at the dynamical heart of the theory, rather than being something that needs to be added on as an extra "bridging principle".] These matters are peripheral to the central focus of this letter, which is my claim that, as regards "bridging principles", consciousness is very different from the subject matters of the "physical sciences" , which I interpret broadly as including biology. I claim that---insofar as the classical physics theoretical framework is regarded as providing the correct basic physical description---bringing the physical facts about the structure and functioning of biological systems (including human body/brains) into the physical theory requires no bridging principle analogous to those required to relate consciousness to the physical theory. Because my thoughts on this are, I think, essentially orthodox, I had previously thought it unnecessary to spell them out in detail. But you are probably rejecting something I regard as obvious, so I must give you enough rope to hang me (or to pinpoint where you think I go wrong), even if that means repeating an old story. It all boils down, in my thinking, to the Bohr- Einstein debate, and to the essential notion there that, in Bohr's words, "the language classical physics is an extension of everyday language", which we use to "communicate to others what we have learned and what we have done". This connection of classical physics to everyday language goes way back. Just to hit the highlights, there is the beginning of language in prehistoric times when grunts and cries gave way to words, some of which were tied to physical objects of interest, like sticks and stones etc.. Some important steps on the road to classical physics were: (1) the development of "numbers", a whole conceptual development involving written and verbal symbols, and the linkage of manipulations of these symbols to inventories (bags of grain) in Babylonia and Egypt; (2) Geometry, which, as its name signifies, was used by the Egyptians in measuring out plots of land (earth); (3) Application of ideas from geometry to observations of stars: astronomy (Aristarchus of Samos, Copernicus, Kepler) (4) Connection of ideas from geometry and astronomy to terrestial dynamics, i.e., to objects such as projectiles and falling apples etc., (Galileo, Newton). This development involves the creation of a conceptual framework that allows human beings to communicate to each other what they have seen and what they have done, and that links their words pertaining to these matters to a conceptual framework based on geometry and ideas from astronomy, generalized, however, to "tiny planets" ( i.e., particles) from which the objects we see and act upon are conceived to be built. Since the time of Newton tens of thousands of scientists, technicians, and engineers have been developing this gigantic interconnected conceptual framework linking all sorts of descriptions of "what we see and what we have done" to this central conceptual notion that the objects that we "can see" and "can handle" are built up out of these "tiny planets" that obey laws similar to Newton's laws of motions of the "observed planets". Words like "thermometer", "barometer" ,"strain gauge", "microscope", "dog", "cat", "frog", "cell", etc. come to have meanings to groups of communicating human beings, in the sense that clear examples of objects in the classes specified by such words can be recognized by the persons that belong to these groups by virtue of their indoctrination (i.e., training). Two of the primary things that large groups of people learn to recognize are the written and oral symbols for the integers. The physical sciences try to be quantitative, and to tie into empirical findings that involve numbers. Readings on clocks and rulers have been central to physics since the time of Galileo's experiments on objects rolling down inclined planes, and even long before if astronomical data are included. Trained observers can recognize, generally, whether or not some mark on an object lies between the markings labeled by the integers N and N+1 on a ruler or clock. Using more sophisticated measuring devices they can recognize, and communicate to others, which number appears as the visually displayed result of some measurement. It is an essential feature of good experimental work in the physical sciences that the results of the experiments be such that they can be recognized by trained observers to fall into one of a set distinguishable alternatives that are describable in the technical language of the discipline. In meteorology there are possible measurements of temperature, pressure, humidity, wind velocities, etc,. at various times and places. Given a possible classical theoretical description of the world (a description of the locations of all the particles for all times, and the values of all the fields for all times) the numbers displayed on all of the measuring devices will certainly be determined (i.e., specified, entailed). By virtue of what has been said regarding the connection of the language used by the scientist to what they can see and do, the empirical findings constitute an "objective" (independent of which member of the group is making the observation) characterization of the system. This communicable characterization carves out a particular subset of the set of all possible classical worlds. There is *entailment* here, in the sense that, for example, the basic physical description entails the observed results of the measurements of the temperatures, pressures, humidities, wind velocities, etc., at the various spacetime locations. Conversely, a possible set of readings restricts the set of possible worlds. Certain features of biological systems are analogous to the features of meteorological systems mentioned above. A frog is placed on a scale and the displayed weight is 8.3 ounces. Another frog is wired up, and electrical responses to various stimuli are displayed. Another frog has been dissected and a certain cell is identified by the researcher. He has been trained to be to do this by his professor of cell biology: recognizing a cell is in principle similar to recognizing a rattle-snake, or the number "8". This cell is measured to be between 5.3 and 5.4 mm in length. Another researcher measures the rate of cell division in a certain sample of cells that has been prepared in a specified way. Classical physical theory is a theory, and as such it exists in the realms of experiences of scientists and technicians. These realms of experience contain not only these conceptual elements but also experiences that a scientist might call "experiences about the physical world coming via their senses". Classical physical theory is based upon certain correspondences that exists between these two kinds of experiences, which we can call physical/theoretical and physical/empirical experiences, respectively. The physical/theoretical concepts were constructed as generalizations of the physical/empirical experiences, ("observed planets" being the models for "tiny planets") so the two classes of experiences are of the same general kind, and hence can be compared. The matching between physical/theoretical experience and physical/empirical experience, on the basis of recognizable spacetime forms, is of course the basis of the physical sciences. The "bridging principle" connecting the physical/theoretical experiences to the physical/empirical experiences is, however, nothing but the fact that by suitable indoctrination we can be taught to recognize spacetime forms belonging to various named classes. But the two things compared are of the same kind: they are both "experiences of spacetime forms". The subject matters of these physical sciences discussed above have been things like planets, falling apples , billiard balls, hurricanes, frogs, cells observed in vivo and in vitro, etc.. They are things we observe mainly through our eyes, perhaps with the aid of physical measuring devices such as telescopes, microscopes, thermometers, barometers, clocks, and rulers, etc.. For each of us, his own the physical/empirical experiences of, say, seeing an "8 or a "dog" or a "cells" or a "female woman" is imbedded in a larger realm of experiences that contain, over time, feelings of joy, frustration, sorrow, warmth, hunger, etc. An important feature of classical physics is precisely that it is silent about all of these further experiential-type qualities or properties of the physical systems that it describes. The complete basic physical description of any physical system is in terms of the particles and fields, and the full set of physical properties consists exactly of what is entailed by these basic facts. Classical physics never introduces the idea of what an apple "feels" as it falls, yet claims to entail everything about how this apple, and everything else, behaves without including any such variable, or quality, or property. So if our brains are included in the classical physics theoretical description of the physical world then there will not appear in this classical description of these brains anything beyond the descriptions of the spacetime dispositions of the the particles and fields of which they are built, and all properties entailed by these microscopic facts. You apparently agree that this entailment does not include our consciousness experiences. You say that physics does not entail biology. A distinction is needed here. The discussion given above shows, I think, that--- within the classical physical conceptualization---the complete classical physics description of the world would describe all the physical properties of biological systems, such as being built from cells that divide, and that have cell walls with certain physical properties , etc.. This description would also give in principle a complete causal description of the dynamics of living systems. In this sense, the classical physics description would---to the extent that it is true---give a complete physical description of all the physical facts about biological systems. Of course, these physical facts about biological systems do not include people's *opinions* about such systems. This complete-in-principle physical description might fall short of what biologists want: the failure of the physical theory to be able to compute in practice the detailed behaviours of biological systems might render the basic physical theory inadequate for their practical purposes. Thus they might be moved to append to the classical physical conception of biological systems some extra concepts that are not entailed by the basic physical description. This "practical biology" would then be on a par with the science of consciousness in the sense that both would entail adding concepts and/or properties that go beyond what the classical conception entails. This observation about what I have called "practical biology" [to distinguish it from "physical biology", which is the collection of biological facts entailed by the (classical) physical description of nature] is, I think, not pertinent to the present discussion. In the first place, it does alter the fact that the classical physics description *does not entail* the existence of consciousness in normally behaving human beings. That was my main claim (which you now seem to endorse as obvious, an opinion that I would not gainsay). But that fact, coupled with the fact that the classical description certainly *does entail* the physical facts about behaviours, means that any consciousness that does happen to be present is ineffective: human behaviour would be the same whether it was present or not. That circumstance was the basis of my opinion that the classical conceptualization of the basic physics was unsatisfactory. I am not clear whether or not we are in agreement on this point, and, if not, where and why. In the second place, it does not alter the fact that within the classical physics conceptualization of nature there is, in connection with bridging principles, a huge difference between the basic facts that the science of biology deals with and the basic facts that science of consciousness must accommodate. The basic facts of biology are the facts that there are cells that divide, and that have cell walls with certain properties, that DNA exists and has certain physical properties, etc.. These kinds of facts are the kinds of facts that, according to the classical physics conceptualization of nature, are entailed by the physical facts. And the bridging principles that are needed to connect these biological facts about some system that is being observed or contemplated to the basic facts of the classical-physics description of that system are essentially trivial, being simply the ability of trained scientists to: (1), recognize matchings between spacetime forms that belong to various classes, such as "8's" and "cells" (which communities of communicating scientists have singled out as worth identifiying); and (2), communicate to other members of their community the existence of these matchings. But the bridging principles needed to link, generally, the conscious experiences associated with various brain activities to the basic physical facts about these activities, as they are described in classical physics, are not of this trivial kind. This big difference arises because in the case of the subject matters of the physical sciences the bridging principles connect two things of the same kind: they connect, typically, just two things, an observer's direct experience of seeing a spacetime form, and his conception of seeing a spacetime form that is built out of spacetime forms similar to things he can see, but are merely smaller. This connection links the physical facts about a system as they are represented in the theory to the physical facts about that system as they are empirically experienced by someone who is observing the system. But in the case of the science of consciousness the situation is quite different. There is, first of all, a brain that can be both observed and conceptualized by various "outside" observers. At this level everything is like meteorology and biology. But, *in addition to* the experiencings of this brain by the various outside observers, there is another kind of experiencing associated with this brain, namely the experiencings by the person to whom the brain belongs. This latter kind of experience might include a "deep sorrow". The connection between experiening a "deep sorrow" and experiencing a "spacetime form" is not like the connection between experiencing two structurally similar spacetime forms. Even if a person could eventually be trained to "see" a certain spacetime activity in a brain as representing a "deep sorrow" the situation would still not be like the study (within the classical conceptualization) of a brain as simply a biological/physical system. For the brain considered as a classically conceived biological/physical system we would consider only those experiences associated with the brain that come through people's sense organ's (primarily the eye and the closely connected spacetime theoretical concepts, whereas the consideration of a feeling of "deep sorrow" associated with this brain pertains to experiences known only through another channel of access to the properties of this brain. In the third place, according to the classical conceptualization of nature, any aspects of a biological system that are not physical facts, or entailed by the basic physical facts, are necessarily experiences belonging to some realm of experiences. For these are the only kinds of things that there are in the classical conceptualization of nature. So anything that any extra biological bridging principles (for "practical biology") brings in that are not entailed by the physical facts should be identified as just experiences in some person's or people's minds. But all such experiences are supposed to be entailed by the "bridging principle" associated with consciousness. But then no extra bridging principles beyond those needed for conscious experiences are needed for biology. This is merely a brief sketch of what I believe to be essentially the orthodox classical view of things: I do not think I have exposed any view unfamiliar to you. But perhaps my laying the elements of classical thinking out in this way will allow you to quickly identify the source or sources of our inability to communicate with each other. Best regards, Henry