Non-Euclidean Geometry

"[...] non-Euclidean geometry is by no means intended to decide the validity of the parallel axiom, but only whetherthe parallel axiom is a mathematical consequence of the remaining axioms of Euclid; a question to which these investigations give a definite no. Because [...] these remaining axioms suffice to construct a system of theories which includes Euclidean geometry merely as a special case." (Felix Klein, 1873)

"[...] the illustration of a space of constant positive measure of curvature by the familiar example of the sphere is somewhat misleading. Owing to the fact that on the sphere the geodesic lines (great circles) issuing from any point all meet again in another definite point, antipodal, so to speak, to the original point, the existence of such an antipodal point has sometimes been regarded as a necessary consequence of the assumption of a constant positive curvature. The projective theory of non-Euclidean space shows immediately that the existence of an antipodal point, though compatible with the nature of an elliptic space, is not necessary, but that two geodesic lines in such a space may intersect in one point if at all." (Felix Klein, "The Most Recent Researches in Non-Euclidian Geometry", [lecture] 1893)

"Euclidean geometry can be easily visualized; this is the argument adduced for the unique position of Euclidean geometry in mathematics. It has been argued that mathematics is not only a science of implications but that it has to establish preference for one particular axiomatic system. Whereas physics bases this choice on observation and experimentation, i. e., on applicability to reality, mathematics bases it on visualization, the analogue to perception in a theoretical science. Accordingly, mathematicians may work with the non-Euclidean geometries, but in contrast to Euclidean geometry, which is said to be intuitively understood," these systems consist of nothing but 'logical relations' or 'artificial manifolds'. They belong to the field of analytic geometry, the study of manifolds and equations between variables, but not to geometry in the real sense which has a visual significance." (Hans Reichenbach, "The Philosophy of Space and Time", 1928)

"The steady progress of physics requires for its theoretical formulation a mathematics which get continually more advanced. […] it was expected that mathematics would get more and more complicated, but would rest on a permanent basis of axioms and definitions, while actually the modern physical developments have required a mathematics that continually shifts its foundation and gets more abstract. Non-Euclidean geometry and noncommutative algebra, which were at one time were considered to be purely fictions of the mind and pastimes of logical thinkers, have now been found to be very necessary for the description of general facts of the physical world. It seems likely that this process of increasing abstraction will continue in the future and the advance in physics is to be associated with continual modification and generalisation of the axioms at the base of mathematics rather than with a logical development of any one mathematical scheme on a fixed foundation." (Paul A M Dirac, "Quantities singularities in the electromagnetic field", Proceedings of the Royal Society of London, 1931)

"Conventionalism as geometrical and mathematical truths are created by our choices, not dictated by or imposed on us by scientific theory. The idea that geometrical truth is truth we create by the understanding of certain conventions in the discovery of non-Euclidean geometries." (Clifford Singer, "Engineering a Visual Field", 1955)

"[...] gradually and unwittingly mathematicians began to introduce concepts that had little or no direct physical meaning. Of these, negative and complex numbers were most troublesome. It was because these two types of numbers had no 'reality' in nature that they were still suspect at the beginning of the nineteenth century, even though freely utilized by then. The geometrical representation of negative numbers as points or vectors in the complex plane, which, as Gauss remarked of the latter, gave them intuitive meaning and so made them admissible, may have delayed the realization that mathematics deals with man-made concepts. But then the introduction of quaternions, non-Euclidean geometry, complex elements in geometry, n-dimensional geometry, bizarre functions, and transfinite numbers forced the recognition of the artificiality of mathematics." (Morris Kline, "Mathematical Thought from Ancient to Modern Times", 1972)

"An instance of such a 'non-Euclidean' geometry is projective geometry, concerned with those properties of figures which do not change under projective transformations. Projective geometry is not merely not Euclidean geometry; it is 'very much non-Euclidean'." (Isaak Yaglom, "Geometric Transformations", 1973)

"The mathematicians who originally conceived of non-euclidean geometry, Bolyai, Lobachevsky and to some extent Gauss, seem all to have conceived of the theory as one which is potentially applicable to physical space. [...] The original BL [Bolyai-Lobachevsky, or hyperbolic] geometers saw their results as holding for the case of a single parallel or for the case of multiple parallels. Because of this, the issue facing the pioneers of BL geometry was not strictly speaking consistency but truth. It was the question of whether the possibilities they envisioned of multiple non-intersecting lines were ever realized." (Michael J Scanlan, "Beltrami’s model and the independence of the parallel postulate", History and Philosophy of Logic 9, 1988)

"There is weirdness in non-Euclidean geometry, but not because of anything that geometers might say about the ordinary fond familiar world in which space is flat, angles sharp, and only curves are curved. Non-Euclidean geometry is an instrument in the enlargement of the mathematician’s self-consciousness, and so comprises an episode in a long, difficult, and extended exercise in which the human mind attempts to catch sight of itself catching sight of itself, and so without end." (David Berlinski, "Infinite Ascent: A short history of mathematics", 2005)