(Intellect:) Sweet is by convention and bitter by convention, cold by convention, colour by convention; in truth there are but atoms and the void.
(The Senses:) Wretched mind, from us you are taking the evidence by which you would overthrow us? Your victory is your own fall.
DEMOCRITUS, fr. 125.
I should not be a genuine theoretician if I were not the first to ask: what is theory? The layman observes in the first place that theory is difficult to understand and surrounded with a tangle of formulae that to the uninitiated speak no language at all. However they are not its essence, the true theoretician uses them as sparingly as he can; what can be said in words he expresses in words, while it is precisely in books by practical men that formulae figure all too often as mere ornament.
I am of the opinion that the task of the theory consists in constructing a picture of the external world that exists purely internally and must be our guiding star in all thought and experiment; that is in completing, as it were, the thinking process and carrying out globally what on a small scale occurs within us whenever we form an idea.
One is almost tempted to assert that quite apart from its intellectual mission, theory is the most practical thing conceivable, the quintessence of practice as it were, since the precision of its conclusions cannot be reached by any routine of estimating or trial and error; although given the hidden ways of theory, this will hold only for those who walk them with complete confidence.
LUDWIG BOLTZMANN, ON THE SIGNIFICANCE OF THEORIES
... I will not quarrel over 'reality', at least not until I have to. 'Reality', 'existence', and so forth are empty words. All that matters to me is this: suppose you do see it as necessary to refer to the broadly shared character of one part of our experience (the part we call external) to the idea that same die applied to similar 'malleable surfaces' will tend to produce similar structures, you still ought not to suppose that this can either explain or guarantee our awareness of this shared character. If you suppose that there is a real external world which is the active cause of sensation and which can consequently be influenced by our voluntary actions (and idea which I recommend you to avoid), then the danger arises of going on from this plausible explanation of our common experience to regarding our knowledge of it as obvious, inevitable and complete, and to stop being concerned about its origin or the degree of completeness to which it can attain. To do so is simply wrong; this is no mere dispute over words.
ERWIN SCHRÖDINGER, MY VIEW OF THE WORLD
The reciprocal relationship of epistemology and science is of noteworthy kind. They are dependent upon each other. Epistemology without contact with science becomes an empty scheme. Science without epistemology is - insofar as it is thinkable at all - primitive and muddled. However, no sooner has the epistemologist, who is seeking a clear system, fought his way through to such a system, than he is inclined to interpret the thought-content of science in the sense of his system and to reject whatever does not fit into his system. The scientist, however, cannot afford to carry his striving for epistemological systematic that far. He accepts gratefully the epistemological conceptual analysis; but the external conditions, which are set for him by the facts of experience, do not permit him to let himself be too much restricted in the construction of his conceptual world by the adherence to an epistemological system. He therefore must appear to the systematic epistemologist as a type of unscrupulous opportunist: he appears as realist insofar as he seeks to describe a world independent of the acts of perception; as idealist insofar as he looks upon the concepts and theories as the free inventions of the human spirit (not logically derivable from what is empirically given); as positivist insofar as he considers his concepts and theories justified only to the extent to which they furnish a logical representation of relations among sensory experiences. He may even appear as Platonist or Pythagorean insofar as he considers the viewpoint of logical simplicity as an indispensable and effective tool of his research.
ALBERT EINSTEIN, in P.A. Schilpp, ALBERT EINSTEIN: PHILOSOPHER-SCIENTIST
If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generation of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis that
All things are made of atoms-little particles that that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another.
In that one sentence, you will see, there is an enormous amount of information about the world, if just a little imagination and thinking are applied.
RICHARD FEYNMAN, LECTURES NOTES ON GRAVITATION
...conventional formulations of quantum theory, and of quantum field theory in particular, are unprofessionally vague and ambiguous. Professional theoretical physicists ought to be able to do better. Bohm has shown us a way.
JOHN BELL, SPEAKABLE AND UNSPEAKABLE IN QUANTUM MECHANICS
JOHN BELL, AGAINST "MEASUREMENT"
If the electron is a point, it is a material point, and thus differs from points in empty space. The difference does not consist in anything characteristic of the electron at an instant, but in its causal laws. What distinguishes a material point from a point of empty space-time is that we recognize a series of earlier and later material points as all parts of the history of one electron. In the Newtonian theory, one could say the same of a point of absolute space; but with the abandonment of absolute space we have become unable to regard a point at one time as in any sense the same as a point at another time, except in the case of a material point. The existence of this connection may be taken as the definition of "matter", and obviously the connection is causal.
BERTRAND RUSSELL, ANALYSIS OF MATTER.
There will thus remain a certain sphere which will be outside physics. To take a simple instance; physics might, ideally, be able to predict that at such a time my eye would receive a stimulus of a certain sort; it might be able to trace the physical properties of the resulting events in the eye and the brain, one of which is, in fact a visual percept; but it could not itself give us knowledge that one of them is a visual percept. It is obvious that a man who can see knows things which a blind man cannot; but a blind man can know the whole of physics. Thus the knowledge that other men have and he has not is not part of physics.
BERTRAND RUSSELL, ANALYSIS OF MATTER.
Physics is mathematical, not because we know so much about the physical world, but because we know so little: it is only its mathematical properties that we can discover. For the rest our knowledge is negative.
BERTRAND RUSSELL, AN OUTLINE OF PHILOSOPHY.
... for example, heat is naturally construed as the cause of heat sensations. Does this mean that we have to explain heat sensations before we can explain heat? Of course we have no good account of heat sensations (or of experience generally), so what happens is practice is that that part of the phenomenon is left unexplained. ... To be sure, no explanation of heat will be complete until we have an account of how that causal connection works, but the incomplete account is good enough for most purposes. It is somewhat paradoxical that we end up explaining almost everything about a phenomenon except for the details of how it affects our phenomenology, but it is no problem in practice. It would not be an happy state of affairs if we had to put the rest of science on hold until we had a theory of consciousness.
DAVID CHALMERS, THE CONSCIOUS MIND
A reductionist thinks that science is about analyzing 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. Complexity prevents us from using fundamental physics to make high-level predictions, so instead we guess what those predictions would be if we could make them-- emergence gives us a chance of doing that successfully-- and supposedly that is what the higher-level sciences are about. Thus to reductionists and instrumentalists, who disregard both the real structure and the real purpose of scientific knowlege, the base of the predictive hiearchy of physics is by definition the 'theory of everything.' But to everyone else scientific knowledge consists of explanations, and the structure of scientific explanations does not reflect the reductionist hierarchy. There are explanations at every level of hierarchy. Many of them are autonomous, referring only to concepts at that particular level (for instance, 'the bear ate the honey because it was hungry'). Many involve deductions in the opposite direction to that of reductive explanation. That is, they explain things not by analyzing them into smaller, simpler things but by regarding them as components of larger, more complex things-- about which we nevertheless have explanatory theories. For example, consider one particular copper atom at the tip of the nose of the statue of Sir Winston Churchill that stands in Parliament Square in London. Let me try to explain why that copper atom is there. It is because Churchill served as prime minister in the House of Commons nearby; and because his ideas and leadership contributed to the Allied victory in the Second World War; and because it is customary to honor such people by putting up statues of them; and because bronze, a traditional material for such statues, contains copper, an so on. Thus we explain a low-level physical observation-- the presence of a copper atom at a particular location-- through extremely high-level theories about emergent phenomena such as ideas, leadership, war and tradition.
There is no reason why there should exist, even in principle, any lower-level explanation of the presence of that copper atom than the one I have just given. Presumably a reductive 'theory of everything' would in principle make a low-level prediction of the probability that such a statue will exist, given the condition of (say) the solar system at some earlier date. It would also in principle describe how the statue probably got there. But such descriptions and predictions (wildly infeasible, of course) would explain nothing. They would merely describe the trajectory that each copper atom followed from the copper mine, through the smelter and the sculptor's studio, and so on. They could also state how those trajectories were influenced by forces exerted on surrounding atoms, such as those compromising the miners' and the sculptor's bodies, and so predict the existence and shape of the statue. In fact such a prediction would have to refer to atoms all over the planet, engaged in the complex motion we call the Second World War, among other things. But even if you had the superhuman capacity to follow such lengthy predictions of the copper atom's being there, you would still not be able to say, 'Ah yes, now I understand why it is there.' You would merely know that its arrival there in that way was inevitable (or likely, or whatever), given all the atoms' initial configurations and the laws of physics. If you wanted to understand why, you would still have no option but to take a further step. You would have to inquire into what it is about that configuration of atoms, and those trajectories, that gave them the propensity to deposit a copper atom at this location. Pursuing this inquiry would be a creative task, as discovering new explanations always is. You would have to discover that certain atomic configurations support emergent phenomena such as leadership and war, which are related to one another by high-level explanatory theories. Only when you knew those theories could you understand fully why that copper atom is where it is.
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.
DAVID DEUTSCH, THE FABRIC OF REALITY.