AXIOM OF REDUCIBILITY
(1) Godel uses the axiom of reducibility axiom 1V of his system is the axiom of reducibility “As Godel says “this axiom represents the axiom of reducibility (comprehension axiom of set theory)†(K Godel , On formally undecidable propositions of principia mathematica and related systems in The undecidable , M, Davis, Raven Press, 1965,p.12-13)
. Godel uses axiom 1V the axiom of reducibility in his formula 40 where he states “x is a formula arising from the axiom schema 1V.1 ((K Godel , On formally undecidable propositions of principia mathematica and related systems in The undecidable , M, Davis, Raven Press, 1965,p.21
“ [40. R-Ax(x) ≡ (∃u,v,y,n)[u, v, y, n <= x & n Var v & (n+1) Var u & u Fr y & Form(y) & x = u ∃x {v Gen [[R(u)*E(R(v))] Aeq y]}]
x is a formula derived from the axiom-schema IV, 1 by substitution “ mrob.com/pub/math/goedel.html
what godel calls the axiom of reducibility is his streamlined version of
russells axiom
math.ucla.edu/~asl/bsl/1302/1302-001.ps.
"The system P of footnote 48a is Godel’s
streamlined version of Russell’s theory of types built on the natural
numbers as individuals, the system used in [1931]. The last sentence ofthe footnote allstomindtheotherreferencetosettheoryinthatpaper;
KurtGodel[1931,p. 178] wrote of his comprehension axiom IV, foreshadowing
his approach to set theory, “This axiom plays the role of [Russell’s]
axiom of reducibility (the comprehension axiom of set theory).â€
from the collected works of godel volume 3
godel states 1939
books.google.com/books?id=gDzbuU … #PPA119,M1
“to be sure one must observe that the axiom of reducibility appears in
different mathematical systems under different names and forms”
he is noting AR has different forms
IT MUST BE NOTED THAT GODEL IS USING 2ND ED PM BUT RUSSELL TOOK THE AXIOM OF REDUCIBILITY OUT OF THAT EDITION – which Godel must have known.
The Cambridge History of Philosophy, 1870-1945- page 154
books.google.com/books?id=I09hCI … _RmOLy_JS0
Quote
“In the Introduction to the second edition of Principia, Russell repudiated Reducibility as 'clearly not the sort of axiom with which we can rest content’…Russells own system with out reducibility was rendered incapable of achieving its own purposeâ€
quote page 14
helsinki.fi/filosofia/gts/ramsay.pdf.
“Russell gave up the Axiom of Reducibility in the second edition of
Principia (1925)â€
Phenomenology and Logic: The Boston College Lectures on Mathematical Logic and Existentialism (Collected Works of Bernard Lonergan) page 43
books.google.com.au/books?id=Pd5 … 6QrI&hl=en
“In the second edition Whitehead and Russell took the step of using the simplified theory of types dropping the axiom of reducibility and not worrying to much about the semantical difficultiesâ€
Godels paper is called
ON FORMALLY UNDECIDABLE PROPOSITIONS
OF PRINCIPIA MATHEMATICA AND RELATED
SYSTEMS
but he uses an axiom that was not in PRINCIPIA MATHEMATICA thus his proof/theorem has nothing to do with PRINCIPIA MATHEMATICA AND RELATED SYSTEMS at all
Godels proof is about his artificial system P -which is invalid as it uses the ad hoc invalid axiom of reducibility
Godel constructs an artificial system P made up of Peano axioms and axioms including the axiom of reducibility- which is not in the edition of PM he says he is is using. This system is invalid as it uses the invalid axiom of reducibility. Godels theorem has no value out side of his system P and system P is invalid as it uses the invalid axiom of reducibility
As a corollary Godel did not destroy the Hilbert Frege Russell programme to create a unitary deductive system in which all mathematical truths can be deduced from a handful of axioms
Godel is said to have shattered this programme in his paper called “On formally undecidable propositions of Principia Mathematica and related systems” but this paper it turns out had nothing to do with “Principia Mathematica†and related systems" but instead with a completely artificial system called P Godel uses axioms which where not in his version of PM thus his proof/theorem cannot apply to PM thus he cannot have destroyed the Hilbert Frege Russell programme and also his system P is artificial and applies to no system anyways
( 2) “As a corollary, the axiom of reducibility was banished as irrelevant to mathematics … The axiom has been regarded as re-instating the semantic paradoxes†- mind.oxfordjournals.org/cgi/repr … 28/823.pdf
2)“does this mean the paradoxes are reinstated. The answer seems to be yes and no†- fds.oup.com/www.oup.co.uk/pdf/0-19-825075-4.pdf )
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It has been repeatedly pointed out this Axiom obliterates the distinction according to levels and compromises the vicious-circle principle in the very specific form stated by Russell. But The philosopher and logician FrankRamsey (1903-1930) was the first to notice that the axiom of reducibility in effect collapses the hierarchy of levels, so that the hierarchy is entirely superfluous in presence of the axiom.
(helsinki.fi/filosofia/gts/ramsay.pdf)
-
Russell Ramsey and Wittgenstein regarded it as illegitimate Russell abandoned this axiom – in 2nd ed PM- and many believe it is illegitimate and must be not used in mathematics
Ramsey says
Such an axiom has no place in mathematics, and anything which cannot be
proved without using it cannot be regarded as proved at all.
This axiom there is no reason to suppose true; and if it were true, this
would be a happy accident and not a logical necessity, for it is not a
tautology. (THE FOUNDATIONS OF MATHEMATICS* (1925) by F. P. RAMSEY
the standford encyclopdeia of philosophy says of AR
plato.stanford.edu/entries/princ … thematica/
“many critics concluded that the axiom of reducibility was simply too ad hoc to be justified philosophicallyâ€
From Kurt Godels collected works vol 3 p.119
books.google.com/books?id=gDzbuU … #PPA119,M1
“the axiom of reducibility is generally regarded as the grossest philosophical expediency “
Godel would have know all these criticism by Russell Wittgenstein and Ramsey but still used the axiom. Russell Witgenstein and Ramsey would have know Godel used this invalid axiom in his artificial system P but said nothing