Ideals of Proof (IP): An Interdisciplinary Project

Michael Detlefsen, Senior chaire d'excellence, ANR, France 2007--2011

Supporting Agencies, Institutions & Departments

Chaire de philosophie du langage et de la connaissance (Jacques Bouveresse)


The term 'ideals' in the title is used in two senses. The first concerns the aims and virtues of proof considered as justificative norms for mathematical practice generally. The second concerns the use of so-called "ideal" elements or methods as means of pursuing these aims.

Ideals in the first sense include not only such traditional standards as rigor, certainty, apriority, purity and explanatory gain, but also such "collective" or systemic virtues as (various types of) completeness, closure, efficiency and freedom. Generally speaking, we want to improve our understanding of why such conditions and constraints as have figured as ideals of proof in the history of mathematics have so figured and whether they are truly deserving of such regard.

Ideals in the second sense include such things as the introduction of "infinites" (both large and small), imaginary and complex numbers in algebra and analysis, the use of Kummer ideals in number theory and the use of points, lines and planes at infinity in projective geometry.

We're concerned with ideals in both of the above senses. We're also interested in the relationships between them and are especially concerned to determine how the use of ideals in the second sense may either support or run contrary to realization of the ideals in the first sense. More generally we want to identify and understand the contributions ideal elements or methods have made and may yet make to the larger enterprises of mathematical proof and knowledge.

One example of the type of question with which we're concerned comes from the frequently encountered claim that the use of ideal elements stands to improve our efficiency as problem-solvers and theorem-provers. The following remark by Hadamard illustrates the view.

"… the shortest and best way between two truths of the real domain often passes through the imaginary one."

"An Essay on the Psychology of Invention in the Mathematical Field'' (1945)

Such claims raise many questions. Are they, for example, intended to suggest that the use of ideal elements in some sense affords gains (either of quality, extent or efficiency) in the attainment of mathematical knowledge? And does this further imply that solutions and theorems produced through application of ideal methods are to be taken as genuinely adding to our knowledge, and that they do so while consuming fewer resources than alternative methods?

Another example comes from the common claim that the distinction between real and ideal elements in mathematics is confused or implausible and ought to be abandoned. The following remark by the 20th century American mathematician James Pierpont is typical of this view.

"… all numbers are equally real and equally imaginary. Historically ... the term imaginary still clings to the complex numbers; pedagogically we must deplore using a term which can only create confusion"

Functions of a complex variable (1914)

Like Hadamard's remark, this remark too raises certain questions. Among these is that concerning whether the several stages of the successive extension of the number-concept are indeed alike or whether there are significant differences between them. Each stage of extension requires the abrogation of certain theorems. Are the theorems relinquished at one stage of the same basic character and centrality as those relinquished at others? If there are differences, how important are they, and what do they signify? If, for example, one stage of extension were to require relinguishment of more basic or important theorems than another, would this be evidence of a difference in the degree or character of the imaginariness or ideality between them?

These are but two illustrations of inquiries belonging to the IP project. Others concern (i) how the use of ideal elements/methods may affect the pursuit of rigor and such higher epistemic goals as purity and explanatory adequacy, (ii) what consequences admission of ideal elements/methods might have for such things as the role of constructive reasoning in proofs, (iii) what type(s) of freedom the use of ideal elements and methods represents and what place the exercise of such freedom(s) rightly has in proofs and other forms of mathematical reasoning, (iv) the relationship between the use of ideal elements and methods and various ideals of completeness or closure and (v) what effects admission of ideal methods in proof might have on the "logic" of the notions of mathematical proof and provability.

IP is an interdisciplinary initiative intended to bring researchers from a variety of disciplines together to achieve a better understanding of that distinctive higher human cognitive function that is mathematics. We welcome inquiries and proposals from scientists and scholars of all ranks and disciplines who believe they have something to contribute to the project. More information may be obtained by clicking on the buttons for the various subprojects.

We thank the French Agence Nationale de la Recherche (ANR) for their generous financial support.


IP includes subprojects, more of which shall be added progressively according to the proposals of participants. Detailed descriptions of existing subprojects are available, in HTML or PDF :

Subproject I: Ideal Elements and the Increase of Resolutory Capacity

Subproject II: Ideal Elements and Construction

Subproject III: Ideals of Completeness

Subproject IV: Ideal Elements and Ideals of Efficiency



Patrick Blackburn, INRIA Lorraine, France  [homepage]

Jacques Bouveresse, Collège de France, Chaire de Philosophie du langage et de la connaissance, France  [homepage]

Michael Detlefsen, University of Notre Dame, USA  [homepage]

Gerhard Heinzmann, Département de Philosophie, Nancy-Université (U. Nancy 2), Archives Henri Poincaré, France  [homepage]

Philippe Nabonnand, Département de Mathématiques-Informatique, Nancy-Université (U. Nancy 2), Archives Henri Poincaré, France [homepage]

Marco Panza, CNRS, REHSEIS, Département d’Histoire et de Philosophie des Sciences, Université  Paris-Diderot, Paris 7, France  [homepage]

David Rabouin, CNRS, REHSEIS, Département d’Histoire et de Philosophie des Sciences, Université  Paris-Diderot, Paris 7, France  [homepage]

Manuel Rebuschi, Département de Mathématiques-Informatique, Nancy-Université (U. Nancy 2), Archives Henri Poincaré, France  [homepage]

Ivahn Smadja, Département d’Histoire et de Philosophie des Sciences, REHSEIS, Université  Paris-Diderot, Paris 7, France  [homepage]

Jean-Jacques Szczeciniarz, Département Histoire et Philosophie des Sciences, REHSEIS, Université Paris-Diderot, Paris 7, France  [homepage]

Participating researchers

Distinguished senior fellows

Pr. Timothy McCarthy Gödel's Incompleteness Theorems and Ideals of Completeness


Post-doctoral fellows


Dr. Andrew Arana Proof and Extraneous Information
Dr. Paola Cantù Ideal Numbers and Magnitudes
Dr. Renaud Chorlay The reception of Riemann-surfaces in complex function theory
Dr. Walter Dean Formal Models of Mathematical Knowledge and Justification
Dr Sébastien Maronne Ideal elements in Geometry in the 1650’s: the Pascalian program
Dr. Paul McCallion Are All Numbers Equally Real?
Dr. John Mumma Ideal Elements & Geometrical Knowledge
Dr. Andrei Rodin Forcing and Ideal Elements
Dr. Fabien Schang Acts of Reasoning
Dr. Frédérick Tremblay Proofs and rationality


Doctoral fellows

Mattia Petrolo The Metamorphosis of Logical Constructivity
Sean Walsh Hume's Principle and Mathematical Induction