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Bertrand Russell 18th May 1872 – 2nd February 1970 Early history Born in 1872 into one of the most prestigious families in Britain. Both parents died before he was five years old Raised by a staunchly Presbyterian Grandmother Educated by various tutors but introduced to Euclid’s works by his brother. (video) Went to Cambridge in 1890 His problems with Euclid’s Elements (1902) • Proposition 1 – “an assumption not noticed by Euclid because of the dangerous habit of using a figure” • Proposition 4 – “a tissue of nonsense” • Proposition 6 – missing an axiom • Proposition 7 – “so thoroughly fallacious that Euclid would have done better not to have attempted a proof” http://www-history.mcs.st-and.ac.uk/Extras/Russell_Euclid.html Proposition 1 It is required to construct an equilateral triangle Describe the circle BCD with centre A and radius AB. Again describe the circle ACE with centre B and radius BA Join the straight lines CA and CB from the point C at which the circles cut one another to the points A and B. Now, since the point A is the centre of the circle CDB then AC equals AB. Since the point B is the centre of the circle CAE therefore BC equals BA But AC was proved equal to AB therefore each of the straight lines AC and BC equals AB And things which equal the same thing also equal one another, therefore AC also equals BC Therefore the three straight lines AC, AB and BC equal one another. Therefore the triangle ABC is equilateral and it has been constructed on the given finite straight line AB. Proposition 4 If two triangles have two sides equal to two sides respectively, and have the angles contained by the equal straight lines equal, then they also have the base equal to the base, the triangle equals the triangle, and the remaining angles equal the remaining angles respectively, namely those opposite the equal sides. D A B C E F AB and AC equal DE and DF respectively and angle BAC equals EDF Superimpose ABC on DEF and all points, lines and angles coincide. Proposition 6 If in a triangle two angles equal one another, then the sides opposite the equal angles also equal one another. Let ABC be a triangle where angle ABC equals angle ACB I say that the side AB also equals the side AC. If AB does not equal AC, then one of them is greater. Let AB be greater. Cut off DB from AB the greater equal to AC the less, and join DC. Since DB equals AC, and BC is common, therefore the two sides DB and BC equal the two sides AC and CB respectively, and the angle DBC equals the angle ACB. I.4 C.N.5 Therefore the base DC equals the base AB, and the triangle DBC equals the triangle ACB, the less equals the greater, which is absurd. Therefore AB is not unequal to AC, it therefore equals it. Proposition 7 Given two straight lines constructed from the ends of a straight line and meeting in a point, there cannot be constructed from the ends of the same straight line, and on the same side of it, two other straight lines meeting in another point and equal to the former two respectively, namely each equal to that from the same end. “Many more general criticisms might be passed on Euclid's methods, and on his conception of Geometry; but the above definite fallacies seem sufficient to show that the value of his work as a masterpiece of logic has been very grossly exaggerated.” B Russell, The Teaching of Euclid, The Mathematical Gazette 2 (33) (1902), 165-167. Russell’s Paradox 1902: Gottlob Frege - Grundgesetze der Arithmetik (Basics of Arithmetic) Volume II For any formula 𝑃(𝑥) where 𝑥 can be anything then there exists a set {𝑥: 𝑃 𝑥 } In other words if T is the property of being a teacup then the set of all teacups S is 𝑆 = {𝑥: 𝑇 𝑥 } We could also have the set of all “non-teacups” Which contains itself Russell’s Paradox Call the set of all sets that are not members of themselves “R.” If R is a member of itself, then by definition it must not be a member of itself. Similarly, if R is not a member of itself, then by definition it must be a member of itself. A book about all books which don’t refer to themselves Books which refer to themselves Books which don’t refer to themselves Principles of mathematics (1903) “In spite of its five hundred pages the book is much too short.” - G.H. Hardy “[It] turned out to be a crude and rather immature draft of the subsequent work [Principia Mathematica]” – Russell (1959) Principia Mathematica • Was an attempt to describe a set of symbolic axioms and rules from which all mathematics was derived. • Unrestricted creation of sets is no longer allowed. • Theory of Types. Russell’s Paradox and the Theory of Types • All men in a town must be clean shaven • Men must either shave themselves or be shaved by the barber • The (male) barber is only allowed to shave those who don’t shave themselves. • What does the barber do? Russell’s Paradox and the Theory of Types • Don’t let anyone shave themselves • Form several castes in society • Caste 1 shaves Caste 2 • Caste 2 shaves Caste 3 etc. • Caste 1 remains unshaven A proof from Principia Mathematica Volume I, page 372 Dot notation Dots are effectively brackets 1 + 2 × 3 + 4 × (5 + 6) 1 + 2. × .3 + 4.× 5 + 6 1+2 ×3 +4 1 + 2. × 3 ∶ + 4 Proof of 1+1 ( 𝛼, 𝑏 ∈ 1 ⇒ 𝛼 ∩ 𝛽 = ∅ ≡ 𝛼 ∪ 𝛽 ∈ 2) ) 1 and 2 have been clearly defined by this stage ( 𝛼, 𝑏 ∈ 1 ⇒ 𝛼 ∩ 𝛽 = ∅ ≡ 𝛼 ∪ 𝛽 ∈ 2) This is simply a statement of a previous proposition that has already been proved 𝛼 = 𝑥 ,𝛽 = 𝑦 ⇒ 𝛼 ∪ 𝛽 ∈ 2 ≡ 𝑥 ≠ 𝑦 And by another previously proved theorem (if x and y are different then…) 𝑥 ∩ 𝑦 =∅ And by another previously proved theorem (the two generic sets 𝛼 and 𝛽) 𝛼∩𝛽 =∅ 11.11 and 11.35 just let you split a property into two and prove it separately "Conclusion (1), with propositions ∗11.11 and ∗11.35, implies that if ∃ 𝑥, 𝑦 and 𝛼 = {𝑥}, 𝛽 = {𝑦} ⇒ 𝛼 ∪ 𝛽 ∈ 2 ≡ 𝛼 ∩ 𝛽 = ∅ These two theorems just say that you can prove things separately and then put them back together to prove the whole Putting it all together ( 𝛼, 𝑏 ∈ 1 ⇒ 𝛼 ∩ 𝛽 = ∅ ≡ 𝛼 ∪ 𝛽 ∈ 2) 𝛼 = 𝑥 ,𝛽 = 𝑦 ⇒ 𝛼 ∪ 𝛽 ∈ 2 ≡ 𝑥 ≠ 𝑦 ≡ 𝑥 ∩ 𝑦 =∅ ≡𝛼∩𝛽 =∅ (1) And if ∃ 𝑥, 𝑦 and 𝛼 = {𝑥}, 𝛽 = {𝑦} ⇒ 𝛼 ∪ 𝛽 = 2 ≡ 𝛼 ∩ 𝛽 = ∅ (2) By (1) and (2) the proposition has been demonstrated Actual proof of 1+1=2 Vol II, page 86 Gödel's incompleteness theorem (1931) For any system of consistent axioms, there will always be statements about the natural numbers that are true but unprovable in the system. Any system like this cannot demonstrate its own consistency Russell’s life after PM Spent time in prison for opposing WW1 Moved to America – lost his position at the University of Chicago over his opinions on sexual morality Eventually agreed that conflict opposing Nazi Germany was necessary during WW2 Survived a plane crash (1948) Spent post war period opposing nuclear armament Nobel prize for Literature in 1950 Jailed for 7 days (age 89) for his part in an anti-nuclear demonstration Died of Influenza in 1970 Further “reading” Closing message from Bertrand Russell Clip from https://www.youtube.com/watch?v=1bZv3pSaLtY )