The relationship between the philosophy of nature and modern science is complex. Opinions vary from an outright rejection by scientists of this branch of philosophy to the recognition of the inadequacy of science. While in the study of nature a philosophical approach prevailed until the end of the seventeenth century, ever since modern physics and chemistry have imposed themselves as the only valuable study of nature. Aristotle's cosmological system had turned out to be wrong. His theory of weight as a factor with regard to the speed of falling bodies had been refuted by experiments. Since his physics were linked to his philosophy of nature, scientists failed to distinguish between these two disciplines and swept both from the table. They believed that all process in the world could be expressed in mathematically framed formulas. Their arguments seemed so powerful and the discoveries they made so convincing that even some Thomists began to think that the properties of things studied in the philosophy of nature do not allow us to reach any valuable knowledge of material reality. Objective is what can be tested and measured. What cannot is relegated to the domain of the subjective. The study of the "how" replaced that of the "what" and the "why". Arguing in this vein the Dominican D.H. Salman wrote that the perceptible qualities (qualitates sensibiles) do not possess the ontological importance which they have been given in former times: it has now become clear that we must traverse this zone of what is perceived by the senses (the physically impure) to reach the authentic and objective world. Only the sciences, Salman believed, give access to the nature of physical things. If so, the philosophy of nature can start only from what has been established by the sciences(1).
However, father Norbert Luyten protested against this position. Salman's view is mistaken. If one rejects the spontaneous experience of things as the starting-point of philosophical reflection, one undermines the certainty of all knowledge and is well underway to scepticism. The philosophy of nature aims at attaining the intelligibility of things, a knowledge which differs from the functional understanding which the sciences provide(2). Salman replied that Luyten is speaking about something subjective. Perception is subjective. Moreover, what we preceive is ontologically insignificant.
We think that Luyten's position is correct. One must keep in mind that the description of the material world and of living beings which the sciences give us is as far removed from the representation by common sense as the picture proposed by the ancient myths. I may perhaps recall that Claude Lévi-Strauss believed that the sciences and myths play a comparable role, that is to say both try to give an answer to the demands of the mind for coherence and explanation. In this sense it is not true that modern science has destroyed all of the philosophical understanding of the world.
So much about the conflict situation between classical physics and the philosophy of nature as it existed until after World War I. In recent years the rapid progress of natural science and the splendid results it obtained, in particular in quantum physics, cosmology and biochemistry, have brought about considerable changes. However, what makes a discussion of the problems by scientists and philosophers difficult is the absence of clear definitions and of a perception of the basic principles involved on the side of the physicists. It appears that scientists often follow an unproven creed, such as "everything comes from matter", "there is no design", "the more proceeds from the less". Even quantum mechanics has from its birth been contaminated by a subjectivits epistemology(3).
Since one of the papers to be read during this seminar will deal with quantum physics I shall make only a few observations. Quantum physics is believed to introduce us into the totally different world of a plethora of sub-atomic particles and unpredictable movements. In the quantum world the principle of causality would no longer apply and certain laws, such as that of the maximum velocity being the speed with with light is propagated, seem to admit exceptions. Paul Davies asserts that the central truth of quantum physics is that of the loss of the relationship between cause and effect. Reality as described in quantum physics is not -independent of the observer. To measure is to act upon reality.
To this we may add the impact of the rapid development of biochemistry, the discovery of genes and the genetic code, the claim that living organisms are the outcome of a lotery. According to Jacques Monod, one of the main protagonists of this theory, the appearance of life and of man does not result from design, but from chance(4). It is also argued that all classes of living beings have similiar genes, but in different arrangments so that the various classes of animals have been constituted with the same components(5).
However, in recent years some scientists have argued that one of the most significant facts, although unnoticed by many, is the emergence of a new paradigm, that is a new way of thinking, which holds that the world is not self-sufficient or self-explaining: there is a mystery behind the smallest particles. Science is not able to give an absolute explanation of reality. The twentieth century has revealed to us the limits of the scientific method. E. Schrödinger states that the particles are only images, but that we cannot do without them(6). According to K. Popper scientific theories are human inventions. They are like nets by which we try to catch the world, they are not like photographs. Moreover they must all the time be corrected. It is no longer the exception that a scientist assumes that, behind the expansion of the universe and the development of life, there is a source of being and energy of which we can dimly discern some traces. This different outlook has been brought about by the discovery of the limits imposed on our knowledge, the irreducible incertitude of quantum physics, the ultimate inaccessibility of material reality and perhaps also by the observation of certain phenomena inexplicable in the framework of standard concepts: one photon is able to pass simultaneously through two fissures; particles stay in contact when going into different directions, even at a certain distance, etc.
To throw some light on the issue and to see better the role of the philosophy of nature I have reduced the main probems to five questions :
(1) What to say about the profusion of particles such as hadrons, meson, photons, baryons, hyperons, nucleons, neutrinos, electrons, neutrons, positrons, etc. and their derivations from quarks and leptons? The meaning of the wave particle structure of particles also belongs to this set of questions.
(2) What to think of time as the fourth dimension and Einstein's claim that space and time are a function of matter?
(3) About matter and form and substance as underlying the phenomena.
(4) Are mass and energy transformed into each other?
(5) What to say about the DNA as the cause of life? Is a random formation of genes possible?
(Ad 1) At the beginning of the twentieth century scientists believed the world to consist of small bodies moving through space according to mathematically expressable laws. They supposed that nothing exists outside the mind which cannot be reduced to these entities and formulated in such laws. All causality and process are reducible to motion, the deplacement of bodies in space and time or to the emission of energy. Obviously the atomic theory rests upon observed facts and atoms have a certain reality. This applies also to subatomic particles. In this connection one may mention the Brownian movement, the emission of light by atoms and molecules and the fact that in chemical reactions the elements combine in fixed proportions of weight. On the basis of these and similar facts certain scientists were convinced that electrons, photons and other particles are as real as stones and trees. According to them the world does not consist of those gross bodies which we believe to see but of an enormous number of extremely small particles, each of which possesses a certain amount of energy and acts on other particles. The mass of such particles as the electrons has even be determined. The main thrust of this scientific thinking was reductionist: reality would be nothing else but a collection of atoms, pictured as small balls.
Already early in the twentieth century two leading philosophers, Bergson and Husserl, pointed out the rather relative value of the scientific knowledge of nature. Physicists, they said, do not see much further than the assumptions they use(7) and throw a veil over our direct experience of reality. As I have said above, the discovery of energy quanta, the formulation of the general and special theory of relativity and the birth of quantum physics caused the downfall of the classical theory of space, bodies and determinism. Several scientists now began to speak of atoms and particles as symbolic representations which are not the things themselves(8). It has become an almost general conviction of those who reflect on the meaning of scientific theories that such theories present each time a hypothetical model to explain the phenomena. They give an image of the world with the help of certain assumptions, are not just based on observed facts but are also a product of imagination(9). Mechanism which is now considered by most as totally antiquated is such a scheme.
It collapsed by the discovery of new facts. Experiments made clear that matter and energy show a dual character. They appear as a continuous whole or as independent particles, according to the nature of the experiment to which they are submitted. Some physicists, e.g. D. Bohm, introduced the concept of wholeness and that of a non-local aspect of elementary matter. In this holistic approach electrons are regarded no longer as independent particles but as aspects of a global situation; they are open to information coming to them from the whole(10). According to others the basic reality of corporeal things does not consist of particles, but of strings, that is finite sequences in which the corporeal substances are substituted by functions. Some believe that the different explanations concern each time only an aspect of matter, but that a definite theory will never be reached. The numerous particles within the atom are theoretical constructions of certain activities taking place within the atom(11). Apparently there is a difficulty here. What is exactly perceived? The atomic theory has been elaborated within the frame work of a certain philosophy, sc. materialist mechanism. There is no reason whatsoever to accept this materialism and we should continue holding the doctrine of the essential nature of things according to which the underlying substance of material things is the bearer of their properties and functions. The atomic structure of which the scientists believe to observe certain aspects is not the deepest layer of things and does not make up their essence. It explains very well certain properties and has a functional value, but it does not give us the essence of material things.
With this we have answered the basic question about the considerable number of particles distinguished in quantum physics. The very number of them, - almost thirty according to some - , suggests that one should not imagine the atom to be composed of as many separate entities, but that there is rather question of different functions.
(Ad 2) Einstein rejected the classical view of time. He could show that an absolute measuring of time is impossible because all fixed points, all standards are continuously changing. He even wanted to do away with the distinction between the past, present and future. In his theory time is a function accompanying bodies in movement. The moment when something is observed or measured has to be introduced into the formula expressing the the observed properties. So Einstein spoke of time as a fourth dimension(12). To a certain extent a Thomist can accept the position of Einstein, insofar as time is not an absolute, but is basically a movement or process as retained by the human mind which distinguished the now from what has been and from what is to come. Time is not a comprehensive entity of its own, within which cosmic processes would take place. Outside the human mind time exists only according to its indivisible instants. Aquinas's position holds the middle between two extremes: the view of time as an absoluted and the reduction of time to an entirely subjective category. In an important observation which is quite close to what some physicists say(13), St. Thomas notes that all movements in the universe are based on one movement which is the origin and cause of all process and change. Therefore, one who observes a change in himself or in the things surrounding him, actually perceives this fundamental mutability and the cosmic process on which it dependa. Therefore, anyone who observes some movement, observes time(14).
Turning now to the question of the reality of space we notice that according to Aristotle material substances are the fundamental reality, not space, but where a mathematical approach dominates or scientists follow their imagination space is considered an absolute. Newton needed absolute space as a framework of reference(15). According to Kant space is an apriori form of our imagination in which the brute signals that reach our senses are clothed. It is true that space is a thing of reason, with a basis in reality, sc. the three-dimensional bodies which are at a certain distance from each other. Aquinas upholds the perception of three-dimensional reality as a primary fact(16). Consequently place is not a corporeal reality, but is predicated of bodies. It is conceived as the outer boundary: each body has a certain distance to the rest of the world. It is not itself a body but it is predicated of it, as an accidental determination. As he had done for time, St. Thomas derives this determining function from the boundary of the universe, sc. the sphere of the first heaven(17). Thomas wants to say that to be locally determined is a fundamental fact prior to the presence of qualities; it is related to the entire universe, a view which is remarkable modern. These texts about time and place show how the philosophy of nature of Aquinas is able to clarify basic categories of modern science.
(Ad 3) Aquinas's philosophy of nature scandalizes scientists and philosophically minded readers alike by its adherence to the doctrine of matter and form. The classical idea of primary matter has been the laughing stock of physicists and is considered a relic of a bygone age, serving no purpose. Let us first notice that by primary matter is meant something entirely different from what the scientists call matter. The materia prima is not itself the stuff which exists, but a factor through which a thing is a body; it is neither being, nor not-being but a component of material substances which explains their mutability into other things. With the concept of primary matter we go to the very limit, the lowest level of reality(18). Its value lies in the fact that it explains substantial change in the world. This has often not been understood(19). In the past few centuries matter was considered to be the perceptible stuff of which physical bodies are made, but not a potentiality(20).
Together with the rejection of primary matter substantial forms also went down the drain. Descartes' criticism concerned certain excesses in the use of the term, but it also resulted from a misunderstanding. He applied a mathematical approach to the study of physical things. Because of the considerable distance he introduced between mind and body he came to consider matter (from which he had banished substantial forms) a substance having extension(21). A passage from his Second Metaphysical Meditation illustrates this new view: if one takes a piece of a honeycomb from its beehive, its wax is cool, solid and smells and tastes of honey; if one taps against it, it makes an audible sound. When one heats it, however, not much else but a lump of material is left and one can forget about the qualities perceived by the five senses whereas the wax itself remains. This shows, Descartes concludes, that the deeper nature of material things is just extension ("... nihil aliud quam extensum quid, flexibile, mutabile"). Obviously Descartes'example not only does not prove anything against the doctrine of substantial forms, but it even confirms it: the lump of wax is not empty extension but remains wax with its properties, even if it has lost some of the sensible qualities it had before. One cannot take away these essential properties without making the wax itself disappear. This demands that there is a subject which is the bearer of these properties, sc. the substance called wax, which is marked by its substantial form, as are other material substances.
This substantial form or essence is not identical with a three-dimensional body but is a reality of a deeper order. It is defined as that which exists by itself and is not perceptible by the senses but can only be conceived by the intellect. It is remarkable that in the later developments of quantum physics observations point to an underlying reality which is somehow in control of the behaviour of the particles as observed in certain experiments: particles appear to communicate at a speed higher than that of propagation of light; particles which interacted before remain united by a bond which is neither material nor a form of energy. John Bell's theorem proves the existence of an invisible, deep non-local reality. Bell's discovery has been called the most important achievement in reality research since the invention of the quantum theory.
Recently professor Jérôme Lejeune proposed the following argument on behalf of the reality of substantial forms: "If we consider the complex mechanism of coded molecules which transmit heredity, we notice that at no moment in time the link between parents and their off-spring is of a material nature. We know with the highest degree of certitude that no molecule, no atom present in the fertilezed egg, has the slightest chance of being transmitted to the next generation. Evidently, what is transmitted is a form... of this matter but never this matter itself"(22).
To this we may perhaps add a new problem: 99% of the matter of the universe cannot be observed, but it does have reality, for celestial bodies cannot be entities floating in nothingness; radiation from visible parts of the world is apparently conveyed to us by a medium. Physicists used to speak of the ether as such a medium. H.A. Lorentz asserted that "one of the most important of our fundamental assumptions must be that the ether not only occupies all space between molecules, atoms or electrons, but that it pervades all these particles. We shall add the hypothesis that, though the particles may move, the ether always remains at rest"(23). The hypothesis of a world ether was rejected, but in recent years such scientists as John Bell have been groping for some basic substrate of the various particles.
(Ad 4) In modern physics mass is the force inherent in bodies antagonistic to gravity. Mass is the manner in which a body is related to the acceleration caused by gravity or other forces. Einstein's particular theory of gravity shows the equivalence of mass and energy. In this approach mass no longer signifies a body in its material aspect but the whole of forms of energy a body possesses. This sum becomes smaller when the nuclei of the atoms fall apart and radiation is set free.
However, if by "mass" we understand the substance of things itself, then mass cannot change into energy, which is an accident and a category of being which has nothing to do with "existing in itself and being the support of accidents".
(Ad 5). Paul Davies remarked that "It is ironical that physics which has led the way for all other sciences is now moving toward a more accomodating view of the mind, while the life sciences, following the path of last century physics, are trying to abolish mind altogether"(24). Is life a phenomenon of self-organisation directed to ever more complex states?(25) Or is the evolution of living beings the result of chance and necessity: randon mutations, a lucky turn at a cosmic lotery on the one hand and the statistics of natural selection on the other (Jacques Monod)? Part of the task of a scientist consists in giving a natural explanation of the phenomena for which previously supernatural causes were assumed to be at work. For this reason a number of scientists were inclined to do away with the direct creation by God of the different species of living beings and adhered to Darwin's theory of evolution. which they saw as a comprehensive view. However, philosophy detects several contradictions in this theory. Passing over the difficulties caused by the fossil findings(26) we insist on the fact that according to Darwin no other cause is at work but blind variation. However, blind variation does not explain how beautifully formed organs and organisms come into being. In his monistic materialism Darwin believed that the variations which occur in living beings are generally advantageous, but that this is not intended by nature nor by an extra-mundane cause. However, philosophically speaking this is impossible: chance is not a cause but the absence of it. The theory contradicts what we know from our own experience of the world. The failure to provide a satisfactory explanation of evolution got Darwinism into serious difficulties.
The re-discovery of the laws of Mendel, the rise of molecular biology and the discovery of the genetic code gave rise to Neo-Darwinism and appeared to offer a way out of the difficulties which made classical Darwinism unacceptable. Certain properties of a living being appeared to be linked to particular parts of the genetic code, sc. genes, and can be isolated from other properties. So biologists concluded that contingent mutations in the genetic code located in the nucleus of cells are the carriers of evolution. The discoveries made by biochemistry seemed to reenforce the arguments of the philosophers of the absurd. The different nature and properties of living beings could now be reduced to chemical factors, sc. the complex molecules constituting the genes of the genetic code. However, this hypothesis is also subject to difficulties: mutations appear to concern a rather restricted group of properties which, in the entire living organism, are only a secondary phenomenon and do not bear on the specific heritage. Moreover hundreds of synchronized mutations in the same direction would be required in order to obtain, through mutations in the genetic code, viable forms of life. These concordant and goal-orientated mutations would have to continue during several generations. If they are a product of chance, their synchronized march toward the same goal would not have any explanation. Statistically unexplainable, such a process would be impossible from a philosophical point of view, because at odds with the basic principles of reality..
In addition one should recall that one gene contains an average of 1500 macromolecules, arranged in a determinate order. It would be non-sensical to ascribe such a most precise arrangement to chance. The difficulty becomes even greater if one tries to explain in this way the transition from one species or class of animals to a higher class. The DNA chain of animals belonging to higher classes contains some 10,000 genes more than that of animals on the lower level, all arranged in a particular order. Statistically and philosophically it is impossible to attribute to chance the lengthening of this code with its extremely complex and artful arrangement of molecules(27). The main question is how to explain by chance the lengthening of the genetic code by each time some 10,000 genes in order to obtain a higher class, each gene consisting of well over a thousand macromolecules (nucleotides). Since the different molecules must be arranged in a determinate order to form a gene, it is impossible to attribute this process to chance.
A philosophical attempt at an explanation admires the amazing discoveries of biologists and accepts that many activities of plants and animals must be ascribed to innate programs. It does not deny the contribution of chemical causes, but stresses that the genetic code itself occurs only in living cells and is sustained by the protoplasm of which it is a part. A more basic cause is needed which constitutes this living being as a unity and enables it to go on living even during continuous metabolism. This cause is "the principle of life" (the soul), the individually realized specific nature. It cannot be expressed in terms of chemical elements but this is no reason to discard it as a fiction, unless one wants to believe that the only factors active are chemical reactions. This principle of life is not something besides the cells and the genetic code but it contains, directs and carries them. It is not accessible to a scientific investigation insofar as this proceeds along the lines of a mathematico-quantitative approach. The principle of animal life belongs to the material order but surpasses physico-chemical factors.
The sciences study nature under the aspect of the measurable. In their perspective the efficient and final causes of Aquinas's philosophy have no place. Descartes, Francis Bacon and others did indeed exclude final causes from scientific investigation. Once they had made an option in favour of a certain type of science, sc. to limit themselves to what is quantitatively measurable, their decision was quite natural. But to abstract from final causes does not mean that there are none. A more encompassing philosophical approach goes beyond the level of the quantifiable, discerns the need for a cause to direct and to organize the process of such a development, if indeed there has been evolution.
When we pass to the animals which have sense cognition, the difficulties of a purely physico-chemical explanation become even greater: cognition is an activity sui generis, its explanation by chemical reactions or physical stimuli is entirely inadequate. The theory of evolution is also applied to man to explain his descent from the anthropoids among animals. The ticket "man" would just have appeared in the great lotery of the mutations. Man's so-called spiritual faculties differ only in degree, but not essentially from those of animals. It is true that this critical position with regard to man's uniqueness is not shared by all scientists. Some speak of entirely new categories which made their appearance with man. Who subscribes to a spontaneous development of human life from animal ancestors by chance factors degrades man to just a living being among others, and, if we follow Freud, not even master of himself. Man is alone surrounded as he is by the indifferent immensity of the universe from which he emerged by chance (Jacques Monod). In a world deprived of illusions and light, man experiences himself as a stranger, who has no hope to reach the promised land. S. Weinberg expresses this feeling when he writes that the better we understand the universe, the more we are inclined to think that it is devoid of meaning.
However, also in biology a new way of thinking made its appearance. Convinced of the limits of the physico-chemical explanation by random mutations certain biologists adopt a holist approach and see the physcical world working together to make the appearance of life and of man possible. Ilya Prigogine spreaks of decyphering the story of a new alliance. As a matter of fact, several scientists now consider that the interrelationship of the physical world and living beings is so intricate and processes of life with all the factors involved so complex that it is very unlikely that these systems owe their origin to random events. It would seem, so they argue, that life is directed by design(28). It is suggested that the genetic code must be understood within the framework of a global organisation of time and space. This type of consideration wants science to lead us to awareness of the deeper aspects of reality and to go beyond the simplifications of a merely quantitative approach.
At the end of this survey the indispensable function of the philosophy of nature stands out: it must give an analysis of what we observe. It has a critical task with regard to numerous assertions of contemporary scientists and the attempts to deprive the world of design and man of his unique position. However, a certain measure of humility is befitting not just for scientists but also for philosophers. For many scientists the philosophy of nature is a proud and somewhat ridiculous attempt to speculate about nature without taking into consideration the facts. Time has come to determine with great precision the roll and the limits of both the sciences and the philosophy of nature and to evaluate the conclusions presented by contemporary scientists. It is an encouraging sign that several leading scientists appear to be more open to considerations more in line with the realist philosophy of St. Thomas.
1. See D.H. Salman, "La conception scolastique de la physique", in Revue néoscolastique de philosophie" 39 (1936) 27 - 50.
2. "De actuele stelling van de traditionele natuurphilosophie", in Tijdschrift voor Philosophie 81 (1946) 203 - 225.
3. Cf. Mario Bunge, Philosophie de la physique, Paris 1975, 133.
4. François Mauriac observed that what Monod wants to make us believe is far more incredible than what we, poor catholics, believe.
5. See François Jacob, La souris, la mouche et l'homme, Paris 1997.
6. Mémoires sr la mécanique ondulatoire, Paris 1933, XIV.
7. E. Husserl, Die Krisis der europäischen Wissenschaften.
8. See J. Parrain-Vial, Philosophie des sciences de la nature. Nouvelles tendances, Paris 1983, 246ff.; B. D'Espagnat, A la recherche du réel, Paris 1997.
9. R. Harré, The Philosophies of Science, Oxford 1972, p. 23.
10. See M. Redhead, Incompleteness, Nonlocality and Realism. A Prolegomenon to the Philosophy of Quantum Mechanics, Oxford 1987; N. Herbert, Quantum Reality. Beyond the New Physics, Garden City N.Y. 1987; P.C.V. Davies & J.R. Brown, The Ghost in the Atom. A Discussion of the Mysteries of Quantum Physics, Cambridge 1986.
11. See W. Wallace, "Are Elementary Particles Real?" and " Elementarity and Reality in Particle Physics", in From a Realist Point of View, Lanham/New York/London 1983, 171 - 212. Wallace himself proposes to call these particles real, but virtual, in analogy with the manner in which St. Thomas explains the presence of the elements in chemical compounds. For further information see also W.H. Brock (edit.), The Atomic Debate, Leicester 1967; D.M. Knight, Atoms and Elements, London 1967.
12. See V. Meyer, "Die Zeit in der Relativitätstheorie", in R.W. Meyer, Das Zeitproblem in 20. Jahrhundert, Bern/München 1964, 27 - 43; Correspondance Albert Einstein - Michele Besso, 1903 -1955, Paris 1972.
13. One may think here of the theory of an initial explosion and the ensuing expansion of the universe as the matrix of all cosmic phenomena.
14. In IV Physicorum, lesson 17, 573-574. See also Summa theologiae, I 10, 6.
15. See A.T. Winterbourne, "Newton's Arguments for Absolute Space" in Archiv für Geschichte der Philosophie 67 (1985) 80 - 91.
16. In I Sent., d.25, q.1, a.2: "Ratio corporis in hoc consistit ut in eo possint designari tres dimensiones".
17. In II De anima, lesson 23, n.165: "Vis locantis et continentis derivatur ad elementa ex primo continente, scilicet corpore caelesti".
It follows that outside the world there is neither place nor time. Cf. In I De caelo, lesson 21, n.212.
18. Cf. N. Luyten, "Der begriff der Materia Prima nach Thomas von Aquin", in L.Elders 9ed.), La philosophie de la nature de saint Thomas d'Aquin, Città del Vaticano 1982, 28 - 44.
19. See, e.g., J. Seiler, Philosophie der unbelebten Natur, Olten 1948.
20. See E. McMullin (ed.), The Concept of Matter in Modern Philosophy, Notre Dame 1963.
21. See E. Gilson, Etudes sur le rôle de la pensée médiévale dans la formation du système de Descartes4, Paris 1975, 165ff.
22. L'homme nouveau, April 6, 1997, p. 11.
23. H.A Lorentz, The Theory of Electrons, Leipzig 1909, p.10.
24. God and the New Physics, New York 1984, 8.
25. I. Prigogine, La nouvelle alliance. Métamorphose de la science, Paris 1979, 193.
26. The fossils found in the oldest strata of rocks show a great variety, something which in Darwin's theory is not possible. Moreover, many species appear to have remained the same over tens of thousands of years.
27. This has been convincingly shown by B. Volmert, Das Molekül und das Leben, Hamburg 1985.
28. See Michael Denton, Evolution, une théorie en crise, Paris 1992; id., L'évolution a-t-elle un sens?, Paris 1997.