# The Fabric of Reality **Covers**:: **Source**:: [[The Fabric of Reality by David Deutsch]] **Creator**:: [[David Deutsch]] # Highlights ##### ^287280348 Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=61 ###### ^287280348q If we are to understand the world on more than a superficial level, it must be through those theories and through reason, and not through our preconceptions, received opinion or even common sense. ^287280348 ##### ^287280349 highlight_tags:: [[blue]] Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=99 ###### ^287280349q Being able to predict things or to describe them, however accurately, is not at all the same thing as understanding them. ^287280349 ##### ^287280350 Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=106 ###### ^287280350q Facts cannot be understood just by being summarized in a formula, any more than by being listed on paper or committed to memory. They can be understood only by being explained. Fortunately, our best theories embody deep explanations as well as accurate predictions. ^287280350 ##### ^287280352 highlight_tags:: [[pink]], [[blue]] Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=155 ###### ^287280352q there already is one such oracle out there, namely the physical world. It tells us the result of any possible experiment if we ask it in the right language (i.e. if we do the experiment), though in some cases it is impractical for us to ‘enter a description of the experiment’ in the required form (i.e. to build and operate the apparatus). But it provides no explanations. ^287280352 ##### ^287280353 Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=171 ###### ^287280353q prediction is not the purpose of science, it is part of the characteristic method of science. ^287280353 ##### ^287280354 Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=199 If understanding is explanation then it has never been possible to understand everything, amd likely will never be. ###### ^287280354q Consequently (they say), whether or not it was ever possible for one person to understand everything that was understood at the time, it is certainly not possible now, and it is becoming less and less possible as our knowledge grows. ^287280354 ##### ^287280355 highlight_tags:: [[orange]] Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=202 ###### ^287280355q Physics, for example, has split into the sciences of astrophysics, thermodynamics, particle physics, quantum field theory, and many others. Each of these is based on a theoretical framework at least as rich as the whole of physics was a hundred years ago, ^287280355 ##### ^287471881 Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=216 ###### ^287471881q better explanations in any subject tend to improve the techniques, concepts and language with which we are trying to understand other subjects, and so our knowledge as a whole, while increasing, can become structurally more amenable to being understood. ^287471881 ##### ^287471882 highlight_tags:: [[pink]] Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=223 Their knowledge of twos and ones wouldn't increase, but their knowledge of how Roman's thought might. ###### ^287471882q By learning about Roman numerals, that mathematician would be acquiring no new understanding, only new facts – historical facts, and facts about the properties of certain arbitrarily defined symbols, rather than new knowledge about numbers themselves. ^287471882 ##### ^287471883 highlight_tags:: [[favorite]] Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=238 ###### ^287471883q it is hard to give a precise definition of ‘explanation’ or ‘understanding’. Roughly speaking, they are about ‘why’ rather than ‘what’; about the inner workings of things; about how things really are, not just how they appear to be; about what must be so, rather than what merely happens to be so; about laws of nature rather than rules of thumb. They are also about coherence, elegance and simplicity, as opposed to arbitrariness and complexity, ^287471883 ##### ^287471884 Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=270 ###### ^287471884q Explaining quasars, albeit through existing theories, has given us genuinely new understanding. Just as it is hard to define what an explanation is, it is hard to define when a subsidiary explanation should count as an independent component of what is understood, and when it should be considered as being subsumed in the deeper theory. ^287471884 ##### ^287471885 Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=273 ###### ^287471885q the difference has something to do with creativity. ^287471885 ##### ^287471886 highlight_tags:: [[pink]] Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=291 Perhaps building's don't colapse but they most definitely fail. Shoddy infrastructure is a massive problem, but it isn't befcause of lack of understanding of architecture, it is because of material conditions ###### ^287471886q Nowadays, in contrast, it is quite rare for any structure – even one that is unlike anything that has ever been built before – to fail because of faulty design. ^287471886 ##### ^287471887 Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=328 ###### ^287471887q Thus the issue of whether it is becoming harder or easier to understand everything that is understood depends on the overall balance between these two opposing effects of the growth of knowledge: the increasing breadth of our theories, and their increasing depth. ^287471887 ##### ^287471888 Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=332 ###### ^287471888q We are not heading away from a state in which one person could understand everything that is understood, but towards it. ^287471888 ##### ^287471889 highlight_tags:: [[pink]] Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=334 It feels like Deustch is constructing his defintion of understanding around his goal of being able to understand everything. ###### ^287471889q What I am discussing is the possibility of understanding everything that is understood. ^287471889 ##### ^287471890 highlight_tags:: [[pink]] Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=352 this is a bold claim ###### ^287471890q After the first Theory of Everything, there will be no more great unifications. ^287471890 ##### ^287471891 highlight_tags:: [[pink]] Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=366 ###### ^287471891q It may predict everything (in principle). But it cannot be expected to explain much more than existing theories do, ^287471891 ##### ^287471892 highlight_tags:: [[orange]] Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=391 ###### ^287471892q The reason why higher-level subjects can be studied at all is that under special circumstances the stupendously complex behaviour of vast numbers of particles resolves itself into a measure of simplicity and comprehensibility. This is called emergence: high-level simplicity ‘emerges’ from low-level complexity. High-level phenomena about which there are comprehensible facts that are not simply deducible from lower-level theories are called emergent phenomena. ^287471892 ##### ^288838099 Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=396 ###### ^288838099q The purpose of high-level sciences is to enable us to understand emergent phenomena, of which the most important are, as we shall see, life, thought and computation. ^288838099 ##### ^288838100 highlight_tags:: [[holism]], [[orange]] Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=398 ###### ^288838100q holism – the idea that the only legitimate explanations are in terms of higher-level systems ^288838100 ##### ^288838101 Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=402 ###### ^288838101q A reductionist thinks that science is about analysing things into components. An instrumentalist thinks that it is about predicting things. To either of them, the existence of high-level sciences is merely a matter of convenience. ^288838101 ##### ^288838102 Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=431 ###### ^288838102q In the reductionist world-view, the laws governing subatomic particle interactions are of paramount importance, as they are the base of the hierarchy of all knowledge. But in the real structure of scientific knowledge, and in the structure of our knowledge generally, such laws have a much more humble role. ^288838102 ##### ^288838103 Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=441 ###### ^288838103q There are two theories in physics which are considerably deeper than all others. The first is the general theory of relativity, which as I have said is our best theory of space, time and gravity. The second, quantum theory, is even deeper. ^288838103 ##### ^288838104 Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=450 ###### ^288838104q another way in which reductionism misrepresents the structure of scientific knowledge. Not only does it assume that explanation always consists of analysing a system into smaller, simpler systems, it also assumes that all explanation is of later events in terms of earlier events; in other words, that the only way of explaining something is to state its causes. And this implies that the earlier the events in terms of which we explain something, the better the explanation, so that ultimately the best explanations of all are in terms of the initial state of the universe. ^288838104 ##### ^288838105 highlight_tags:: [[blue]] Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=486 ###### ^288838105q For the cannon-ball, once we have specified, say, the final state it is straightforward to calculate the initial state, and vice versa, so there is no practical difference between different methods of specifying the supplementary data. ^288838105 ##### ^288838106 highlight_tags:: [[blue]] Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=489 ###### ^288838106q But that is exceptional: most of our knowledge of supplementary data – of what specifically happens – is in the form of high-level theories about emergent phenomena, and is therefore by definition not practically expressible in the form of statements about the initial state. For example, in most solutions of the equations of motion the initial state of the universe does not have the right properties for life to evolve from it. Therefore our knowledge that life has evolved is a significant piece of the supplementary data. ^288838106 ##### ^288838107 Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=505 Our perception of reality determines our understanding of it through the abilities and limitations of our intuition. Our understanding is only as good as our margin for error ###### ^288838107q The laws of biology, say, are high-level, emergent consequences of the laws of physics. But logically, some of the laws of physics are then ‘emergent’ consequences of the laws of biology. It could even be that, between them, the laws governing biological and other emergent phenomena would entirely determine the laws of fundamental physics. ^288838107 ##### ^288838122 Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=569 ###### ^288838122q Imagine an electric torch switched on in an otherwise dark room. ... if we were observing the torch from the side we should be able to see neither it nor, of course, its light. Invisibility is one of the more straightforward properties of light. We see light only if it enters our eyes (though we usually speak of seeing the object in our line of sight that last affected that light). ... We cannot see light that is just passing by. If there were a reflective object in the beam, or even some dust or water droplets to scatter the light, we could see where it was. But there is nothing in the beam, and we are observing from outside it, so none of its light reaches us. ^288838122 ##### ^288838126 highlight_tags:: [[pink]] Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=585 ###### ^288838126q That is, a human observer would see nothing; but what about an animal with more sensitive vision? ... If the observer were a frog, and it kept moving ever farther away from the torch, the moment at which it entirely lost sight of the torch would never come. Instead, the frog would see the torch begin to flicker. The flickers would come at irregular intervals that would become longer as the frog moved farther away. But the brightness of the individual flickers would not diminish. At a distance of one hundred million kilometres from the torch, the frog would see on average only one flicker of light per day, but that flicker would be as bright as any that it observed at any other distance. ^288838126 ##### ^288838113 highlight_tags:: [[orange]] Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=602 ###### ^288838113q What happens when a beam of light gets fainter is not that the photons themselves get fainter, but that they get farther apart, with empty space between them ^288838113 ##### ^288838114 highlight_tags:: [[orange]] Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=605 ###### ^288838114q This property of appearing only in lumps of discrete sizes is called quantization. An individual lump, such as a photon, is called a quantum (plural quanta). ^288838114 ##### ^288838115 highlight_tags:: [[pink]] Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=614 ###### ^288838115q FIGURE 2.2 Frogs can see individual photons. ^288838115 ##### ^289554214 highlight_tags:: [[orange]] Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=618 ###### ^289554214q there is also a penumbra between the bright and dark regions: a region which can receive light from some parts of the filament but not from others. ^289554214 ##### ^289554226 Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=618 ###### ^289554226q And because the filament is not a geometrical point, but has a certain size, ... If one observes from within the penumbra, one can see only part of the filament and the illumination is less there than in the fully illuminated, bright region. ^289554226 ##### ^288838117 highlight_tags:: [[orange]] Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=621 ###### ^288838117q However, the size of the filament is not the only reason why real torchlight casts penumbras. The light is affected in all sorts of other ways by the reflector behind the bulb, by the glass front of the torch, by various seams and imperfections, and so on. ^288838117 ##### ^288838118 highlight_tags:: [[orange]] Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=627 ###### ^288838118q FIGURE 2.3 The umbra and penumbra of a shadow. ^288838118 ##### ^288838119 highlight_tags:: [[orange]] Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=639 ###### ^288838119q FIGURE 2.4 Making a narrow beam by passing light through two successive holes.   It turns out that light is not as ductile as gold! Long before the holes get as small as a ten-thousandth of a millimetre, in fact even with holes as large as a millimetre or so in diameter, the light begins noticeably to rebel. Instead of passing through the holes in straight lines, it refuses to be confined and spreads out after each hole. And as it spreads, it ‘frays’. The smaller the hole is, the more the light spreads out from its straight-line path. Intricate patterns of light and shadow appear. We no longer see simply a bright region and a dark region on the third screen, with a penumbra in between, but instead concentric rings of varying thickness and brightness. ^288838119 ##### ^289554216 Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=651 ###### ^289554216q FIGURE 2.5 The pattern of light and shadow formed by white light after passing through a small circular hole. ^289554216 ##### ^289554217 highlight_tags:: [[orange]] Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=684 ###### ^289554217q FIGURE 2.7 The shadows cast by a barrier containing (a) four and (b) two straight, parallel slits. ^289554217 ##### ^289554218 Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=732 ###### ^289554218q Interference is not a special property of photons alone. Quantum theory predicts, and experiment confirms, that it occurs for every sort of particle. ^289554218 ##### ^289554219 Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=735 ###### ^289554219q reality is a much bigger thing than it seems, and most of it is invisible. The objects and events that we and our instruments can directly observe are the merest tip of the iceberg. ^289554219 ##### ^289554220 Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=743 ###### ^289554220q For it turns out that shadow particles are partitioned among themselves in exactly the same way as the universe of tangible particles is partitioned from them. In other words, they do not form a single, homogeneous parallel universe vastly larger than the tangible one, but rather a huge number of parallel universes, ^289554220 ##### ^289554221 highlight_tags:: [[orange]] Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=752 ###### ^289554221q multiverse, has been coined to denote physical reality as a whole. ^289554221 ##### ^289554222 highlight_tags:: [[blue]] Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=767 ###### ^289554222q Indeed, the total density of shadow atoms in even the lightest fog would be more than sufficient to stop a tank, let alone a photon, if they could all affect it. ^289554222 ##### ^289554223 highlight_tags:: [[blue]] Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=788 Is an intuitive universe that you can explain any better than one that you can't? Both require belief ###### ^289554223q As I hope to persuade readers who bear with me, understanding the multiverse is a precondition for understanding reality as best we can. Nor is this said in a spirit of grim determination to seek the truth no matter how unpalatable it may be (though I hope I would take that attitude if it came to it). It is, on the contrary, because the resulting world-view is so much more integrated, and makes more sense in so many ways, than any previous world-view, ^289554223 ##### ^289554224 Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=795 This is what ive always heard ###### ^289554224q ‘A tangible photon is real; a shadow photon is merely a way in which the real photon could possibly have behaved, but did not. Thus quantum theory is about the interaction of the real with the possible.’ ^289554224 ##### ^289554225 Goto: https://readwise.io/to_kindle?action=open&asin=B005KGJX8E&location=800 ###### ^289554225q ‘The possible’ cannot interact with the real: non-existent entities cannot deflect real ones from their paths. ^289554225