Frasi di Roger Penrose

Roger Penrose foto
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Roger Penrose

Data di nascita:8. Agosto 1931

Roger Penrose è un matematico, fisico, cosmologo e filosofo britannico, noto per il suo lavoro nel campo della fisica matematica, in particolare per i suoi contributi alla cosmologia; si occupa inoltre di giochi matematici. Laureato all'Università di Cambridge, è professore emerito all'Istituto di matematica dell'Università di Oxford e nel 1988 ha ricevuto, assieme a Stephen Hawking, il Premio Wolf per la fisica.

Frasi Roger Penrose

„It is a common misconception, in the spirit of the sentiments expressed in Q16, that Godel's theorem shows that there are many different kinds of arithmetic, each of which is equally valid. The particular arithmetic that we may happen to choose to work with would, accordingly, be defined merely by some arbitrarily chosen formal system. Godel's theorem shows that none of these formal systems, if consistent, can be complete; so-it is argued-we can keep adjoining new axioms, according to our whim, and obtain all kinds of alternative consistent systems within which we may choose to work. The comparison is sometimes made with the situation that occurred with Euclidean geometry. For some 21 centuries it was believed that Euclidean geometry was the only geometry possible. But when, in the eighteenth century, mathematicians such as Gauss, Lobachevsky, and Bolyai showed that indeed there are alternatives that are equally possible, the matter of geometry was seemingly removed from the absolute to the arbitrary. Likewise, it is often argued, Godel showed that arithmetic, also, is a matter of arbitrary choice, any one set of consistent axioms being as good as any other.

This, however, is a completely misleading interpretation of what Godel has demonstrated for us. He has taught us that the very notion of a formal axiomatic system is inadequate for capturing even the most basic of mathematical concepts. When we use the term 'arithmetic' without further qualification, we indeed mean the ordinary arithmetic which operates with the ordinary natural numbers 0,1,2,3,4,...(and perhaps their negatives) and not with some kind of 'supernatural' numbers. We may choose, if we wish, to explore the properties of formal systems, and this is certainly a valuable part of mathematical endeavour. But it is something different from exploring the ordinary properties of the ordinary natural numbers. The situation is, in some ways, perhaps not so very unlike that which occurs with geometry. The study of non-Euclidean geometries is something mathematically interesting, with important applications (such as in physics, see ENM Chapter 5 especially Figs 5.1 and 5.2, and also 4.4), but when the term 'geometry' is used in ordinary language (as distinct from when a mathematician or theoretical physicist might use that term), we do indeed mean the ordinary geometry of Euclid. There is a difference, however, in that what a logician might refer to as 'Euclidean geometry' can indeed be specified (with some reservations) in terms of a particular formal system, whereas, as Godel has shown, ordinary 'arithmetic' cannot be so specified.

Rather than showing that mathematics (most particularly arithmetic) is an arbitrary pursuit, whose direction is governed by the whim of Man, Godel demonstrated that it is something absolute, there to be discovered rather than invented (cf. 1.17). We discover for ourselves what the natural numbers are, and we do not have trouble in distinguishing them from any sort of supernatural numbers. Godel showed that no system of 'man-made' rules can, by themselves, achieve this for us. Such a Platonic viewpoint was important to Godel, and it will be important also for us in the later considerations of this book (8.7).“

— Roger Penrose
Shadows of the Mind: A Search for the Missing Science of Consciousness

„At this point, the cautious reader might wish to read over the whole argument again, as presented above, just to make sure that I have not indulged in any 'sleight of hand'! Admittedly there is an air of the conjuring trick about the argument, but it is perfectly legitimate, and it only gains in strength the more minutely it is examined. We have found a computation Ck(k) that we know does not stop; yet the given computational procedure A is not powerful enough to ascertain that facet. This is the Godel(-Turing) theorem in the form that I require. It applies to any computational procedure A whatever for ascertaining that computations do not stop, so long as we know it to be sound. We deduce that no knowably sound set of computational rules (such as A) can ever suffice for ascertaining that computations do not stop, since there are some non-stopping computations (such as Ck(k)) that must elude these rules. Moreover, since from the knowledge of A and of its soundness, we can actually construct a computation Ck(k) that we can see does not ever stop, we deduce that A cannot be a formalization of the procedures available to mathematicians for ascertaining that computations do not stop, no matter what A is.


(G) Human mathematicians are not using a knowably sound algorithm in order to ascertain mathematical truth.

It seems to me that this conclusion is inescapable. However, many people have tried to argue against it-bringing in objections like those summarized in the queries Q1-Q20 of 2.6 and 2.10 below-and certainly many would argue against the stronger deduction that there must be something fundamentally non-computational in our thought processes. The reader may indeed wonder what on earth mathematical reasoning like this, concerning the abstract nature of computations, can have to say about the workings of the human mind. What, after all, does any of this have to do with the issue of conscious awareness? The answer is that the argument indeed says something very significant about the mental quality of understanding-in relation to the general issue of computation-and, as was argued in 1.12, the quality of understanding is something dependent upon conscious awareness. It is true that, for the most part, the foregoing reasoning has been presented as just a piece of mathematics, but there is the essential point that the algorithm A enters the argument at two quite different levels. At the one level, it is being treated as just some algorithm that has certain properties, but at the other, we attempt to regard A as being actually 'the algorithm that we ourselves use' in coming to believe that a computation will not stop. The argument is not simply about computations. It is also about how we use our conscious understanding in order to infer the validity of some mathematical claim-here the non-stopping character of Ck(k). It is the interplay between the two different levels at which the algorithm A is being considered-as a putative instance of conscious activity and as a computation itself-that allows us to arrive at a conclusion expressing a fundamental conflict between such conscious activity and mere computation.“

— Roger Penrose
Shadows of the Mind: A Search for the Missing Science of Consciousness

„In order for A to apply to computations generally, we shall need a way of coding all the different computations C(n) so that A can use this coding for its action. All the possible different computations C can in fact be listed, say as

C0, C1, C2, C3, C4, C5,...,

and we can refer to Cq as the qth computation. When such a computation is applied to a particular number n, we shall write

C0(n), C1(n), C2(n), C3(n), C4(n), C5(n),....

We can take this ordering as being given, say, as some kind of numerical ordering of computer programs. (To be explicit, we could, if desired, take this ordering as being provided by the Turing-machine numbering given in ENM, so that then the computation Cq(n) is the action of the qth Turing machine Tq acting on n.) One technical thing that is important here is that this listing is computable, i. e. there is a single computation Cx that gives us Cq when it is presented with q, or, more precisely, the computation Cx acts on the pair of numbers q, n (i. e. q followed by n) to give Cq(n).

The procedure A can now be thought of as a particular computation that, when presented with the pair of numbers q, n, tries to ascertain that the computation Cq(n) will never ultimately halt. Thus, when the computation A terminates, we shall have a demonstration that Cq(n) does not halt. Although, as stated earlier, we are shortly going to try to imagine that A might be a formalization of all the procedures that are available to human mathematicians for validly deciding that computations never will halt, it is not at all necessary for us to think of A in this way just now. A is just any sound set of computational rules for ascertaining that some computations Cq(n) do not ever halt. Being dependent upon the two numbers q and n, the computation that A performs can be written A(q, n), and we have:

(H) If A(q, n) stops, then Cq(n) does not stop.

Now let us consider the particular statements (H) for which q is put equal to n. This may seem an odd thing to do, but it is perfectly legitimate. (This is the first step in the powerful 'diagonal slash', a procedure discovered by the highly original and influential nineteenth-century Danish/Russian/German mathematician Georg Cantor, central to the arguments of both Godel and Turing.)
With q equal to n, we now have:

(I) If A(n, n) stops, then Cn(n) does not stop.

We now notice that A(n, n) depends upon just one number n, not two, so it must be one of the computations C0, C1, C2, C3,...(as applied to n), since this was supposed to be a listing of all the computations that can be performed on a single natural number n. Let us suppose that it is in fact Ck, so we have:

(J) A(n, n) = Ck(n)

Now examine the particular value n=k. (This is the second part of Cantor's diagonal slash!) We have, from (J),

(K) A(k, k) = Ck(k)

and, from (I), with n=k:

(L) If A(k, k) stops, then Ck(k) does not stop.

Substituting (K) in (L), we find:

(M) If Ck(k) stops, then Ck(k) does not stop.

From this, we must deduce that the computation Ck(k) does not in fact stop. (For if it did then it does not, according to (M)! But A(k, k) cannot stop either, since by (K), it is the same as Ck(k). Thus, our procedure A is incapable of ascertaining that this particular computation Ck(k) does not stop even though it does not.

Moreover, if we know that A is sound, then we know that Ck(k) does not stop. Thus, we know something that A is unable to ascertain. It follows that A cannot encapsulate our understanding.“

— Roger Penrose
Shadows of the Mind: A Search for the Missing Science of Consciousness

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