Geometry (Ancient Greek: γεωμετρία; geo- "earth", -metron "measurement") is a branch ofmathematics concerned with questions of shape, size, relative position of figures, and the properties of space.
The classic example of anaxiomatic system is that ofplane geometry formulated byEuclid... It forms the model of all rigorous mathematical schemes. The axioms are the initial assumptions... From them, logical deductions can proceed under stipulated rules of reasoning... analogous to the scientists'laws of Nature, whilst the axioms play the role ofinitial conditions. We are not free to pick any axioms... They must be logicallyconsistent... Euclid and most other pre-nineteenth-century mathematicians... were also strongly biased towards picking axioms which mirrored the way the world was observed to work... Later mathematicians did not feel so encumbered and have required only consistency from their lists of axioms.
John D. Barrow,Theories of Everything: The Quest for Ultimate Explanation (1991)
Each of five men—Lobachewsky,Bolyai,Plücker,Riemann,Lie—invented as part of his lifework as much (or more) new geometry as was created by all the Greek mathematicians in the two or three centuries of their greatest activity.
The chemist smiles at the childish efforts of alchemists but the mathematician finds the geometry of the Greeks and the arithmetic of theHindoos as useful and admirable as any research of today.
When the value of mathematical training is called in question, quote the inscription over the entrance intothe academy ofPlato, the philosopher: "Let no one who is unacquainted with geometry enter here."
The Egyptians carried geometry no further than was absolutely necessary for their practical wants. The Greeks, on the other hand, had within them a strong speculative tendency. They felt a craving to discover the reasons for things. They found pleasure in the contemplation ofideal relations and loved scienceas science.
TheEudemian Summary says that "Pythagoras changed the study of geometry into the form of a liberal education, for he examined its principles to the bottom, and investigated its theorems in an immaterial and intellectual manner." His geometry was connected closely with his arithmetic. He was especially fond of those geometrical relations which admitted of arithmetical expression.
Aristotle (384-322 B.C.), the systematiser of deductive logic, though not a professed mathematician, promoted the science of geometry by improving some of the most difficult definitions. HisPhysics contains passages with suggestive hints of the principle of virtual velocities. About this time there appeared a work calledMechanica, of which he is regarded by some as the author. Mechanics was totally neglected by the Platonic school.
When Ptolemy once asked Euclid if geometry could not be mastered by an easier process than by studying theElements, Euclid returned the answer, "There is no royal road to geometry."
It is a remarkable fact in the history of geometry, that theElements of Euclid, written two thousand years ago, are still regarded by many as the best introduction to the mathematical sciences.
I would myself say that the purely imaginary objects are the only realities, theὂντως ὂντα, in regard to which the corresponding physical objects are as the shadows in the cave; and it is only by means of them that we are able to deny the existence of a corresponding physical object; and if there is no conception of straightness, then it is meaningless to deny the conception of a perfectly straight line.
The discovery of rigid objects in nature is of fundamental importance. Without it, the concept of measurement would probably never have arisen and metrical geometry would have been impossible. ...As for the physical definition of straightness, it could have been arrived at in a number of ways, either by stretching a rope between two points or by appealing to the properties of these rigid bodies themselves. ...Equipped in this way, the first geometricians (those who built the pyramids, for instance) were able to execute measurements on the earth's surface and later to study the geometry of solids, or space-geometry. Thanks to their crude measurements, they were in all probability led to establish in an approximate empirical way a number of propositions whose correctness it was reserved for the Greek geometers to demonstrate with mathematical accuracy. Thus there is not the slightest doubt that geometry in its origin was essentially an empirical and physical science, since it reduced to a study of the possible dispositions of objects (recognised as rigid) with respect to one another and to parts of the earth. ... Now an empirical science is necessarily approximate, and geometry as we know it to-day is an exact science. It professes to teach us that the sum of the three angles of a Euclidean triangle is equal to 180°, and not a fraction more or a fraction less. Obviously no empirical determination could ever lay claim to such absolute certitude. Accordingly, geometry had to be subjected to a profound transformation, and this was accomplished by the Greek mathematiciansThales,Democritus,Pythagoras, and finallyEuclid. ... But this empirical origin of Euclid's geometrical axioms and postulates was lost sight of, indeed was never even realised. As a result, Euclidean geometry was thought to derive its validity from certain self-evident universal truths; it appeared as the only type of consistent geometry of which the mind could conceive.Gauss had certain misgivings on the matter, but... the honor of discovering non-Euclidean geometry fell toLobatchewski andBolyai.
To-day, thanks toEinstein, we have definite reasons for believing that ultra-precise observation of nature has revealed our natural geometry arrived at with solids and light rays to be slightly non-Euclidean and to vary from place to place. So although the non-Euclidean geometers never suspected it (with the exception ofGauss,Riemann andClifford), our real world happens to be one of the dream-worlds whose possible existence their mathematical genius forsaw.
A. D'Abro,The Evolution of Scientific Thought from Newton to Einstein (1927) p. 37
A more thorough study ofEuclid's axioms and postulates proved them to be inadequate for the deduction of Euclid's geometry. ...Hilbert and others succeeded in filling the gap by stating explicitly a complete system of postulates for Euclidean and non-Euclidean geometries alike. Among the postulates missing in Euclid's list was the celebratedpostulate of Archimedes, according to which, by placing an indefinite number of equal lengths end to end along a line, we should eventually pass any point arbitrarily selected on the line. Hilbert, by denying this postulate, just asLobatchewski andRiemann had denied Euclid'sparallel postulate, succeeded in constructing a new geometry known asnon-Archimedean. It was perfectly consistent but much stranger than the classical non-Euclidean varieties. Likewise, it was proved possible to posit a system of postulates which would yield Euclidean or non-Euclidean geometries of any number of dimensions; hence, so far as rational requirements of the mind were concerned, there was no reason to limit geometry to three dimensions.
A. D'Abro,The Evolution of Scientific Thought from Newton to Einstein (1927) pp. 37-38
Although there is no study which presents so simple a beginning as that of geometry, there is none in which difficulties grow more rapidly as we proceed, and what may appear at first rather paradoxical, the more acute the student the more serious will the impediments in the way of his progress appear. This necessarily follows in a science which consists of reasoning from the very commencement, for it is evident that every student will feel a claim to have his objections answered, not by authority, but by argument, and that the intelligent student will perceive more readily than another the force of an objection and the obscurity arising from an unexplained difficulty, as the greater is the ordinary light the more will occasional darkness be felt. To remove some of these difficulties is the principal object of this Treatise.
[T]he system of concepts of axiomatic geometry alone cannot make any assertions as to the behavior of... practically-rigid bodies. To be able to make such assertions, geometry must be stripped of its merely logical-formal character by the coordination of real objects of experience with the empty conceptual schemata of axiomatic geometry. To accomplish this, we need only add the proposition: solid bodies are related, with respect to their proper dispositions, as are bodies in Euclidean geometry of three dimensions. Then the propositions of Euclid contain affirmations as to the behavior of practically-rigid bodies. Geometry thus completed is evidently a natural science; we may in fact regard it as the most ancient branch of physics. Its affirmations rest essentially on induction from experience... not on logical inferences only. We will call this completed geometry "practical geometry," and shall distinguish it from "purely axiomatic geometry."
Albert Einstein, "Geometry and Experience" (Jan. 27, 1921) Lecture before the Prussian Academy of Sciences, Tr. Alfred Engel, as quoted inThe Collected Papers of Albert Einstein (2002) Vol. 7The Berlin Years: Writings, 1918-1921, pp. 234-235.
Reply given when the rulerPtolemy I Soter asked Euclid if there was a shorter road to learning geometry than through Euclid'sElements.
Attributed toEuclid byProclus (412–485 AD) inCommentary on the First Book of Euclid's Elements as translated by Glenn R. Morrow (1970),p. 57. ἀτραπός "road, trail, track" here takes the more specific sense of "short cut". The Latin translation is by Francesco Barozzi, 1560).
The contemporary decline in interest in geometry and its gradual disappearance from school curricula... should be deplored... Geometry is the most visual of the mathematical disciplines. It is not in principle divorced from numbers, and hence neither is it divorced from algebra. Many a pupil's understanding of algebraic proofs would be considerably reinforced by... visual geometrical proofs which were the hallmark of Greek mathematics and to some extent of Arab mathematics also. ...where a geometrical proof is clear and immediate, as... with... many algebraic identities such as the geometry should not be forgotten. The Greeks were some of the greatest teachers of all time... [and] geometric algebra was in many ways [their] greatest achievement ...
Graham Flegg,Numbers: Their History and Meaning (1983)
The geometrical spirit is not so tied to geometry that it cannot be detached from it and transported to other branches of knowledge. A work of morals or politics or criticism, perhaps even of eloquence, would be better (other things being equal) if it were done in the style of a geometer. The order, clarity, precision and exactitude which have been apparent in good books for some time might well have their source in this geometric spirit. ...Sometimes one great man gives the tone to a whole century; [Descartes], to whom one might legitimately be accorded the glory of having established a new art of reasoning, was an excellent geometer.
Bernard Le Bovier de Fontenelle, "The Utility of Mathematics," i.e. "Préface sur l'utitlité des mathématiques et de la physique et sur les travaux de le Académie des Sciences,"Œuvres de Monsieur de Fontenelle (1753) Vol. 6, pp.37-50, as quoted byHerbert Butterfield,The Origins of Modern Science 1300-1800 (1949).
The way I have taken seems not to lead to the goal, but much rather to make thetruth of geometry doubtful.
Carl Friedrich Gauss (1799) as quoted in the BBC & Open University series, "Topics in the History of Mathematics", MA 290 presented byJeremy Gray,0:39.
Geometry can in no way be viewed... as a branch of mathematics, instead, geometry relates to something already given in nature, namely, space. I... realized that there must be a branch of mathematics which yields in a purely abstract way laws similar to those of geometry.
Hermann Grassmann,Die Lineale Ausdehnungslehre, ein neuer Zweig der Mathematik (1844) [The Theory of Linear Extension, a New Branch of Mathematics] as quoted byMario Livio,Is God a Mathematician? (2009)
I was informed by the priests at Thebes, that kingSesostris made a distribution of the territory of Egypt among all his subjects, assigning to each an equal portion of land in the form of a quadrangle, and that from these allotments he used to derive his revenue by exacting every year a certain tax. In cases however where a part of the land was washed away by the annual inundations of the Nile, the proprietor was permitted to present himself before the king, and signify what had happened. The king then used to send proper officers to examine and ascertain, by admeasurement, how much of the land had been washed away, in order that the amount of tax to be paid for the future, might be proportional to the land which remained. From this circumstance I am of opinion, that Geometry derived its origin; and from hence it was transmitted into Greece.
Herodotus,Histories (c. 450 BC) Book II, c. 109 as quoted by Robert Potts, ed., Introduction toEuclid's Elements of Geometry Book 1-6, 11,12 (1845) p. i.
The authors onTrigonometry may be divided into... theoretical and practical... [N]one... have combined the theory with the practice... to render the subject plain and intelligible... [T]he most valuable and scientifical are... abstruse, and the practical scarcely furnish... the rationale... The object of the ensuing treatise is to simplify the theory, yet to retain a methodical and accurate... investigation, and to exemplify this theory by... important... useful examples. ...[D]emonstrations are frequently founded on principles strictly Geometrical ...and sometimes ...byalgebraical signs, particularly where the Geometrical ...would require a complicated figure, or a ...tedious process. ...[T]he algebraical mode of deduction tends greatly to simplify... yet... definitions and... elementary parts... must be acquired from Geometrical principles illustrated by diagrams; otherwise a student will never obtain a clear and satisfactory knowledge... Should any person attempt to teach the elementary principles of the science by... algebraic characters, and algebraic formulae alone, without the aid of Geometry, he would... deceive both himself and his pupils.
Thomas Keith,An Introduction to the Theory and Practice of Plain and Spherical Trigonometry (1826) Preface,p. iv.
Geometry has two great treasures: one is theTheorem of Phythagoras, the other the division of a line inextreme and mean ratio. The first we can compare to a mass of gold; the other we may call a precious jewel.
Historically, it was Euclidean geometry that, developed to a large extent as a votive offering to the God of Reason, opened men's eyes to the possibility of design and to the possibility of uncovering it by the pursuit of mathematics.
Morris Kline,Mathematics and the Physical World (1959)
The use of canon raised numerous questions concerning the paths of projectiles. ...One might determine... what type of curve a projectile follows and.... prove some geometrical facts about this curve, but geometry could never answer such questions as how high the projectile would go or how far from the starting point it would land. The seventeenth century sought the quantitative or numerical information needed for practical applications, and such information is provided by algebra.
Morris Kline,Mathematics and the Physical World (1959)
Descartes... complained that Greek geometry was so much tied to figures "that is can exercise the understanding only on condition of greatly fatiguing the imagination." Descartes also deplored that the methods of Euclidean geometry were exceedingly diverse and specialized and did not allow for general applicability. Each theorem required a new kind of proof... What impressed Descartes especially was that algebra enables man to reason efficiently. It mechanizes thought, and hence produces almost automatically results that may otherwise be difficult to establish. ...historically it was Descartes who clearly perceived and called attention to this feature. Whereas geometry contained the truth about the universe, algebra offered the science of method. It is... paradoxical that great thinkers should be enamored with ideas that mechanize thought. Of course, their goal is to get at more difficult problems, as indeed they do.
Morris Kline,Mathematics for the Nonmathematician (1967) pp. 255-256.
Over and above the specific theorems created by men such asDesargues,Pascal andLa Hire, several new ideas and outlooks were beginning to appear. The first is the idea ofcontinuous change of a mathematical entity from one state to another... [i.e., of a] a geometrical figure. It wasKepler, in hisAstronomiae Optica of 1604, who first seemed to grasp the fact that parabola, ellipse, hyperbola, circle, and the degenerate conic consisting of a pair of lines are continuously derivable from each other. ...The notion of a continuous change in a figure was also employed by Pascal. He allowed two consecutive vertices of his hexagon to approach each other so that the figure became a pentagon. In the same manner he passed from pentagons to quadrilaterals. The second idea to emerge from the work of theprojective geometers is that oftransformation andinvariance. To project a figure from some point and then take a section of that projection is to transform the figure to a new one. The properties... of interest are those that remain invariant under transformation. Other geometers of the seventeenth century, for example,Gregory of St. Vincent... andNewton, introduced transformations other than projection and section.
Morris Kline,Mathematical Thought from Ancient to Modern Times (1972)
A geometrician has learned to perform the most difficult demonstrations and calculations, as a monkey has learned to take his little hat off and on... All has been accomplished through signs, every species has learned what it could understand, and in this way men have acquired symbolic knowledge...
I claim that many patterns of Nature are so irregular and fragmented, that, compared withEuclid — a term used in this work to denote all of standard geometry — Nature exhibits not simply a higher degree but an altogether different level of complexity … The existence of these patterns challenges us to study these forms that Euclid leaves aside as being "formless," to investigate the morphology of the "amorphous."
Benoît Mandelbrot As quoted in a review ofThe Fractal Geometry of Nature by J. W. Cannon inThe American Mathematical Monthly, Vol. 91, No. 9 (November 1984), p. 594.
I conceived, developed and applied in many areas a new geometry of nature, which finds order in chaotic shapes and processes. It grew without a name until 1975, when I coined a new word to denote it,fractal geometry, from the Latin word for irregular and broken up,fractus. Today you might say that, until fractal geometry became organized, my life had followed a fractal orbit.
If the Greeks had had a mind to reduce mathematics to one field... their only choice would have been to reduce arithmetic to geometry... it is hardly surprising that for nearly two millennia geometry took pride of place in mathematics. And it would have been obvious to any mathematician that a geometrical problem could not be stated or solved in the language of numbers, since the geometrical universe had more structure than the numerical universe. If one desired to translate geometrical problems into the language of numbers,one would have to invent (or discover) more numbers.
Tim Maudlin,New Foundations for Physical Geometry: The Theory of Linear Structures (2014)
Let us calculate the motion of bodies, but also consult the plans of the Intelligence that makes them move. It seems that the ancient philosophers made the first attempts at this sort of science, in looking for metaphysical relationships between numbers and material bodies. When they said that God occupies himself with geometry, they surely meant that He unites in that science the works of His power with the perspectives of His wisdom. From the all too few ancient geometers who undertook such studies, we have little that is intelligible or well-founded. The perfection which geometry has acquired since their time puts us in a better position to succeed, and may more than compensate for the advantages that those great minds had over us.
O king, through the country there are royal roads and roads for common citizens, but in geometry there is one road for all.
Menaechmus (c. 350 BC) response to a request ofAlexander the Great to be taught concisely, as quoted by SirThomas Little Heath,The Thirteen Books of Euclid's Elements (1908)Vol.1Introduction and Books I, II p.1, citingStobaeus,Ecl. (II. p. 228, 30, ed. Wachsmuth)
Thedoctrine of Proportion, in the Fifth Book ofEuclid'sElements, is obscure, and unintelligible to most readers. It is not taught either in foreign or American colleges, and is now become obsolete. It has therefore been omitted in this edition of Euclid'sElements, and a different method of treatingProportion has been substituted for it. This is the commonalgebraical method, which is concise, simple, andperspicuous; and is sufficient for all useful purposes in practical mathematics. The method is clear and intelligible to all persons who know the first principles of algebra. The rudiments of algebra ought to be taught before geometry, because algebra may be applied to geometry in certain cases, and facilitates the study of it.
Geometry is almost the only subject as to which we find truths wherein all men agree; and one cause of this is, that geometers alone regard the true laws of demonstration.
At a very early period the study of Geometry was regarded as a very important mental discipline, as may be shewn from the seventh book of theRepublic ofPlato. To his testimony may be added that of the celebratedPascal (Œuvres,Tom. I. p. 66,) whichMr. Hallam has quoted in hisHistory of the Literature of the Middle Ages. "Geometry," Pascal observes, "is almost the only subject as to which we find truths wherein all men agree; and one cause of this is, that geometers alone regardthe true laws of demonstration." These are enumerated by him as eight in number. 1. To define nothing which cannot be expressed in clearer terms than those in which it is already expressed. 2. Toleave no obscure or equivocal terms undefined. 3. Toemploy in the definition no terms not already known. 4. To omit nothing in the principles from which we argue, unless we are sure it is granted 5. Tolay down no axiom which is not perfectly evident. 6. To demonstrate nothing which is as clear already as we can make it. 7. Toprove every thing in the least doubtful, by means of self-evident axioms, or of propositions already demonstrated. 8. Tosubstitute mentally the definition instead of the thing defined. Of these rules he says, "the first, fourth, and sixth are not absolutely necessary to avoid error, but the otherfive are indispensable; and though they may be found in books of logic, none but the geometers have paid any regard to them.
Various relations being established in geometry between lines constituted under given conditions, as parts of geometrical figures,if we choose to adopt the idea of expressing these lines bynumerical measures, we are then brought to the distinction of such lines being in some casescommensurable in their numerical values, in others not so. Theirgeometrical relations however are absolutely general, and do not refer to any such distinction.
Rev.Baden Powell,On the Theory of Ratio and Proportion (1836).
It is remarkable that thisgeneralization of plane geometry to surface geometry is identical with that generalization of geometry which originated from the analysis of theaxiom of parallels. ...the construction ofnon-Euclidean geometries could have been equally well based upon the elimination of otheraxioms. It was perhaps due to an intuitive feeling for theoretical fruitfulness that the criticism always centered around the axiom of parallels. For in this way the axiomatic basis was created for that extension of geometry in which the metric appears as an independent variable. Once the significance of the metric as the characteristic feature of the plane has been recognized from the viewpoint ofGauss' plane theory, it is easy to point out, conversely, its connection with the axiom of parallels. The property of the straight line as being the shortest connection between two points can be transferred to curved surfaces, and leads to the concept of straightest line; on the surface of the sphere thegreat circles play the role of the shortest line of connection... analogous to that of the straight line on the plane. Yet while the great circles as "straight lines" share the most important property with those of the plane, they are distinct from the latter with respect to the axiom of the parallels: all great circles of the sphere intersect and therefore there are no parallels among these "straight lines". ...If this idea is carried through, and all axioms are formulated on the understanding that by "straight lines" are meant the great circles of the sphere and by "plane" is meant the surface of the sphere, it turns out that this system of elements satisfies the system of axioms within two dimensions which is nearly identical in all of it statements with the axiomatic system ofEuclidean geometry;the only exception is the formulation of the axiom of the parallels. The geometry of the spherical surface can be viewed as the realization of a two-dimensional non-Euclidean geometry:the denial of the axiom of the parallels singles out that generalization of geometry which occurs in the transition from the plane to the curve surface.
Hans Reichenbach (1928, tr. 1957)The Philosophy of Space and Time § 3.
Visual forms are not perceived differently from colors or brightness. They are sense qualities, and the visual character of geometry consists in these sense qualities.
Hans Reichenbach (1928, tr. 1957)The Philosophy of Space and Time § 13.
The influence of geometry upon philosophy and scientific method has been profound. Geometry, as established by the Greeks, starts withaxioms which are (or are deemed to be)self-evident, and proceeds, bydeductive reasoning, to arrive attheorems which are very far from self-evident. The axioms and theorems are held to be true of actual space, which is something given in experience. It thus appeared to be possible to discover things about the actual world by first noticing what is self-evident and then using deduction. This view influencedPlato andKant, and most of the intermediate philosophers... The eighteenth century doctrine ofnatural rights is a search forEuclidean axioms in politics. The form ofNewton'sPrincipia, in spite of its admittedly empirical material, is entirely dominated byEuclid.Theology, in its exact scholastic forms, takes its style from the same source.
The Greeks... discovered mathematics and the art of deductive reasoning. Geometry, in particular, is a Greek invention, without which modern science would have been impossible.
.. general relativity is, of course, based onRiemannian geometry, which is one of the richest frameworks for our understanding of ordinary geometry. ... Many ambitious physicists and mathematicians have gone off into thewilderness in search of some fundamental generalization of geometry that would reconcile gravity with quantum mechanics but, generally speaking, they have come back empty-handed — if at all.
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