# Hans Freudenthal

Hans Freudenthal (September 17, 1905 – October 13, 1990) was a Dutch mathematician. He made substantial contributions to algebraic topology and also took an interest in literature, philosophy, history and mathematics education.

## Quotes

* Educational technique needs a philosophy, which is a matter of faith rather than of science.
** Hans Freudenthal (1977) Weeding and Sowing: Preface to a Science of Mathematical Education. p. 33
* A true aphorism legitimates itself; whoever feels the need to legitimate an aphorism, admits that it is illegal. The surface of an aphorism should conceal profound truth. The claim that everybody can learn everything is superficial, but is as wrong as it can be. As a matter of fact, it is no aphorism but an advertising slogan, and the excuse that it is an aphorism, is a mere wink: in advertising you cannot do without exaggerating. But even as a wink it does not become more true.
** Hans Freudenthal (1977), Weeding and Sowing: Preface to a Science of Mathematical Education, p. 56
* Science should be distinguished from technique and its scientific instrumentation, technology. Science is practised by scientists, and techniques by ‘engineers’ — a term that in our terminology includes physicians, lawyers, and teachers. If for the scientist knowledge and cognition are primary, it is action and construction that characterises the work of the engineer, though in fact his activity may be based on science. In history, technique often preceded science.
** Hans Freudenthal (1978). Weeding and Sowing. Preface to a Science of Mathematical Education; As cited in: Ben Wilbrink (2013) "Hans Freudenthal Aantekeningen bij zijn publicaties".
* [The goal of developmental research is to] consciously experience, describe and justify the cyclic process of development and research so that it can be passed on to others in such a way that they can witness and relive the experience.
** Freudenthal (1988) "Ontwikkelingsonderzoek"; As cited Els Feijs (2005) Constructing a Learning Environment that Promotes Reinvention

### The Concept and the Role of the Model in Mathematics and Natural and Social Sciences (1961)

Hans Freudenthal ed. (1961) The Concept and the Role of the Model in Mathematics and Natural and Social Sciences: : Proceedings of the Colloquium Sponsored by the Division of Philosophy of Sciences of the International Union of History and Philosophy of Sciences of the International Union of History and Philosophy of Sciences Organized at Utrecht, January 1960. D. Reidel Publishing Company.
* No mathematical idea has ever been published in the way it was discovered. Techniques have been developed and are used, if a problem has been solved, to turn the solution procedure upside down, or if it is a larger complex of statements and theories, to turn definitions into propositions, and propositions into definitions, the hot invention into icy beauty. This then if it has affected teaching matter, is the didactical inversion, which as it happens may be anti-didactical. Rather than behaving anti-didactically, one should recognise that the learner is entitled to recapitulate in a fashion of mankind. Not in the trivial matter of an abridged version, but equally we cannot require the new generation to start at the point where their predecessors left off.
** p. ix
* Our mathematical concepts, structures, ideas have been invented as tools to organise the phenomena of the physical, social and mental world. Phenomenology of a mathematical concept, structure, or idea means describing it in its relation to the phenomena for which it was created, and to which it has extended in the learning process of mankind, and, as far as this description is concerned with the learning process of the young generation, it is didactical phenomenology, a way to show the teacher the places where the learner might step into the learning process of mankind.
** p. ix
* No scientist is as model minded as is the statistician; in no other branch of science is the word model as often and consciously used as in statistics.
** p. 79; Part of the article "Models in applied probability", published earlier in Synthese, 12 (1960), p. 204-210.
* No statistician present at this moment will have been in doubt about the meaning of my words when I mentioned the common statistical model. It must be a stochastic device producing random results. Tossing coins or a dice or playing at cards are not flexible enough. The most general chance instrument is the urn filled with balls of different colours or with tickets bearing some ciphers or letters. This model is continuously used in our courses as a didactic tool, and in our statistical analyses as a means of translating realistic problems into mathematical ones. In statistical language " urn model " is a standard expression.
** p. 79; Partly cited in: Norman L. Johnson and Samuel Kotz (1977) Urn Models and Their Application: an. Approach to Modern Discrete Probability Theory, John Wiley & Sons.
* The urn model is to be the expression of three postulates: (1) the constancy of a probability distribution, ensured by the solidity of the vessel, (2) the randomcharacter of the choice, ensured by the narrowness of the mouth, which is to prevent visibility of the contents and any consciously selective choice, (3) the independence of successive choices, whenever the drawn balls are put back into the urn. Of course in abstract probability and statistics the word " choice " can be avoided and all can be done without any reference to such a model. But as soon as the abstract theory is to be applied, random choice plays an essential role.
** p. 80; Cited in: Lev D. Beklemishev (2000) Provability, Computability and Reflection. p. 9

### Realistic models in probability (1968)

Hans Freudenthal (1968) "Realistic models in probability" in: Imre Lakatos (1968) The problem of inductive logic.
* The subject of a science is never well circumscribed and there is little use sharpening its definition. However, nobody will deny that physics deals with nature and sociology with human society in some of their aspects. With logic, it is another matter. Logic is usually understood nowadays as a study of certain formal systems, though in former times there were philosophers who held that the subject matter of logic was the formal rules of human thought. In the latter sense it would be an empirical rather than a formal science, though its empirical subject matter would still be fundamentally different from that of psychology of thinking. One interpretation of logic does not exclude the other. Formal approaches are often easier than empirical ones, and for this reason one can understand why logic as a study of formal systems has till now made more progress than logic as a study of the formal rules of thought, even if restricted to scientific thought.
** p. 1
* The case of methodology is analogous though less clear. Nobody would object to the subject of methodology being science, or some pseudo science. On closer inspection, however, this agreement is no more than a verbal coincidence. It rests on what is meant by science, as reported as a subject of methodology. In fact the subject methodologists call science is more often than not different from what scientists call science. Methodologists are inclined to consider a science as a linguistic system whereas the scientist would only admit that his science has a language, not that it is a language.
** p. 1

### Mathematics as an Educational Task (1973)

Hans Freudenthal (1973). Mathematics as an Educational Task. Dordrecht: D. Reidel.
* The present book is not a methodology of mathematics in the sense that I will systematically show how some teaching matter should taught; it is not even a systematic analysis of subject matter. I hardly ever refer to well-organized classroom experiments evaluated by statistical methods, nor do I cite experimental results of developmental psychology or the psychology of learning. Maybe the most striking feature is that this book contains few quotations. I will try to justify all these features.
** p. v;As cited in: Ben Wilbrink (2013)
* Space and the bodies around us are early mental objects... Name-giving is a first step towards consciousness.
** p. 63; As cited in: Anne Birgitte Fyhn (2007) Angles as Tool for Grasping Space. p. 2
* No doubt once it was real progress when developers and teachers offered learners tangible material in order to teach them arithmetic of whole number... The best palpable material you can give the child is its own body.
** p. 75-76; As cited in: Anne Birgitte Fyhn (2007, p. 6)
* Grasping spatial gestalts as figures is mathematizing of space. Arranging the properties of a parallelogram such that a particular one pops up to base the others on it in order to arrive at a definition of parallelogram, that is mathematizing the conceptual field of the parallelogram.
** p. 133
* The classic instrument to measure drawn angles and to draw angles of a given measure is the protractor — essentially half a circular ring, subdivided by ray segments into 180 degrees. For reasons I was unable to find out, this instrument has recently been superseded by an isosceles right triangle — called geo-triangle, solid, transparant, made of plastic — with an angular division radiating from the midpoint of the hypotenuse to the other sides. Well, inside the triangle half a circle with the midpoint of the hypotenuse as its centre is indicated, and from the position of the degree numbers it becomes clear that it is the semicircle that really matters. One is inclined to say "an outrageously misleading instrument"...
** p. 363
* Euclid defines the angle as an inclination of lines…he meant halflines, because otherwise he would not be able to distinguish adjacent angles from each other… Euclid does not know zero angles, nor straight and bigger than straight angles…Euclid takes the liberty of adding angles beyond two and even four right angles; the result cannot be angles according to the original definitions…Nevertheless one feels that Euclid’s angle concept is consistent.
** p. 476-477
* Angles are measured by arcs, such that 360° and 2π correspond to each other.
** p. 477
* Geometry is grasping space. And since it is about the education of children, it is grasping that space in which the child lives, breathes and moves. The space that the child must learn to know, explore, conquer, in order to live, breathe and move better in it.
** p. 403

### Didactical Phenomenology of Mathematical Structures (1983)

Hans Freudenthal (1983). Didactical Phenomenology of Mathematical Structures. Dordrecht: D. Reidel.
* Learners should be allowed to find their own levels and explore the paths leading there with as much and as little guidance as each particular case requires.
** p. 47

### Revisiting Mathematics Education (1991)

Hans Freudenthal (1991). Revisiting Mathematics Education. China Lectures. Dordrecht: Kluwer Academic Publishers.
* Horizontal mathematising leads from the world of life to the world of symbols.
** p. 41
* [Guided reinvention is] striking a subtle balance between the freedom of inventing and the force of guiding, between allowing the learner to please himself and asking him to please the teacher. Moreover, the learner’s free choice is already restricted by the “re” of “reinvention”. The learner shall invent something that is new to him but well-known to the guide.
** p. 48; As cited in: Anne Birgitte Fyhn (2007, p. 14)
* Vertical mathematising is the most likely part of the learning process for the bonds with reality to be loosened and eventually cut.
** p. 68

### Norbert Wiener (2008)

Hans Freudenthal (2008) "Wiener, Norbert", In Complete Dictionary of Scientific Biography, Charles Scribner’s Sons, Detroit, 2008, pp. 342-347
* In appearance and behaviour, Norbert Wiener was a baroque figure, short, rotund, and myopic, combining these and many qualities in extreme degree. His conversation was a curious mixture of pomposity and wantonness. He was a poor listener. His self-praise was playful, convincing and never offensive. He spoke many languages but was not easy to understand in any of them. He was a famously bad lecturer.
* While studying antiaircraft fire control, Wiener may have conceived the idea of considering the operator as part of the steering mechanism and of applying to him such notions as feedback and stability, which had been devised for mechanical systems and electrical circuits. No doubt this kind of analogy had been operative in Wiener’s mathematical work from the beginning and sometimes had even been productive. As time passed, such flashes of insight were more consciously put to use in a sort of biological research for which Wiener consulted all kinds of people, except mathematicians, whether or not they had anything to do with it. Cybernetics, or Control and Communication in the Animal and the Machine (1948) is a rather eloquent report of these abortive attempts, in the sense that it shows there is not much to be reported. The value and influence of Cybernetics, and other publications of this kind, should not, however, be belittled. It has contributed to popularizing a way of thinking in communication theory terms, such as feedback, information, control, input, output, stability, homeostasis, prediction, and filtering . On the other hand, it also has contributed to spreading mistaken ideas of what mathematics really means
* Even measured by Wiener's standards Cybernetics is a badly organised work — a collection of misprints, wrong mathematical statements, mistaken formulas, splendid but unrelated ideas, and logical absurdities. It is sad that this work earned Wiener the greater part of his public renown, but this is an afterthought. At that time mathematical readers were more fascinated by the richness of its ideas than by its shortcomings.

## External links

* Hans Freudenthal on The MacTutor History of Mathematics archive