This made him in love with geometry.
(Aubrey 1898, 1: 332)
There can be little doubt that this account is embellished,3 but it does comport with some things Hobbes wrote. His Latin Vita reports that in a 1629 journey to the Continent in the company of Sir Gervase Clifton “he began to read Euclid’s Elements; and, well-pleased by its method, not because of the theorems, but rather because of its art of reasoning, he read through it most carefully” (OL I.14). Likewise, the first edition of his Examinatio et emendatio mathematicæ hodiernæ (hereafter Examinatio) contains a passage that closely resembles Aubrey’s account.4 Thus, even if it is unlikely that a chance encounter with Euclid’s statement of the Pythagorean Theorem was literally Hobbes’s first introduction to geometry, there is no doubt that he held the mathematical sciences in high esteem.
What attracted Hobbes to the mathematical “art of reasoning” is the notion that simple and indisputable first principles can lead, by way of rigorous deductions, to important and non-obvious results that are thereby established with demonstrative certainty. He took this as a model for all proper sciences, including the “science of politics,” which he claimed to have founded. There is nothing unique in Hobbes assessment that mathematics provides a model of scientific and philosophical method, but he proposed a philosophy of mathematics that was quite distinctive. In particular, his mathematical ontology rejects the seventeenth century’s received view of the subject and his proposed first principles departed quite significantly from the tradition.
3.1.1 Hobbes’s Mathematical Ontology
A central tenet in Hobbes’s first philosophy is the denial of immaterial substances. As he famously declared in Leviathan chapter 4, “puzzled philosophers” will often “make a name of two names, whose significations are contradictory and inconsistent; as this name, an incorporeall body, or (which is all one) an incorporeall substance” (Hobbes 2012, 60; 1651, 17). This amounts to a declaration that the terms ‘substance’ and ‘body’ are convertible, which entails that any doctrine purporting to deal with a realm of non-physical objects is either arrant nonsense to be dismissed or a confusion standing in need of a fundamental reinterpretation.
In point of fact, mathematics was traditionally understood to be a science whose proper objects are immaterial. The fifth-century ce neo-Platonist Proclus Diadochus summed up the traditional view when he complained that the consideration of matter “muddies” the intellectual apprehension of geometric objects (Proclus 1970, 87). Aristotelians rejected the Platonic concept of a realm of ideal geometric forms, but they nevertheless held that the objects of mathematical investigation must be radically distinguished from the contents of the material world. Christopher Clavius, professor of mathematics at the Jesuit Collegio Romano, summed up the sixteenth-century Aristotelians’ doctrine that mathematics deals with a particular kind of abstract objects that are “considered apart from all matter” or abstracted from any underlying material substrate (Clavius 1612, 1:5). Wallis followed a broadly Aristotelian understanding in his 1657 Mathesis Universalis, proclaiming magnitude to be the object of geometry, adding that “By magnitude I understand lines, surfaces, and bodies taken mathematically, that is, abstracted from matter, whose various affections and properties geometry demonstrates” (Wallis 1693–1699, 1:21).
Where the tradition sought to develop an account of mathematics that makes it independent of the structure or contents of the material world, Hobbes pursued a radically materialistic treatment of the subject. The basis of this program is developed in chapter 8 of De corpore, which bears the title “Of Body and Accident.” In De corpore VIII.4 Hobbes offers a definition of the term ‘magnitude’ with the remark that “The Extension of a Body, is the same thing with the MAGNITUDE of it, or that which some call Real Space” (EW I.105). Having identified magnitude with the extension of body, Hobbes proceeds in De corpore VIII.12 to define the geometric terms ‘point’ and ‘line’ in terms of the central concept of body:
Though there be no Body which has not some Magnitude, yet if when any Body is moved, the Magnitude of it be not at all considered, the way it makes is called a LINE, or one single Dimension; & the Space through which it passeth, is called LENGTH; and the Body it self, a POINT; in which sense the Earth is called a Point, and the Way of its yearly Revolution, the Ecliptick Line.
(EW I.111)
Surfaces and solids are then definable in the same manner. This account of magnitude has an obvious connection to Hobbes’s materialistic ontology. Because he takes only bodies to be real, and because he maintains that it is only through the motion of bodies that anything can be brought about, Hobbes must base all of mathematics on the principles of matter and motion. Mathematical objects must therefore be interpreted as bodies or things produced by the motion of bodies. In particular, the geometric point is a body, which requires that the point be divisible and have magnitude (even if that magnitude is disregarded), while lines, surfaces, and solids arise from the motions of points, lines, and surfaces, respectively.
At first sight, it might seem that there is no place for arithmetic in this scheme. Taking space, motion, and body as fundamental concepts might make tolerable sense of geometry, but it is unclear how this can work for arithmetic. Classical authors drew a firm distinction between the continuous magnitudes of geometry and the discrete “multitudes” of arithmetic. Geometric magnitudes such as lines, angles, surfaces or solids are divisible into lesser magnitudes of the same kind (lines divide into lines, angles into angles, etc.). In contrast, the discrete multitudes of arithmetic were taken to be only finitely divisible and to be ultimately composed from indivisible units. Viewed in this way, geometry and arithmetic are fundamentally different sciences with essentially different objects. Hobbes, however, held that arithmetic could be based on the uniform division of continuous magnitudes into equal parts. As he put the matter, “because any given continuous magnitude can be divided into any number of equal parts, with its ratio to any other magnitude remaining unchanged, it is manifest that arithmetic is contained in geometry” (OL IV.28).
3.1.2 Hobbes on Geometric First Principles
Hobbes’s understanding of geometry as a generalized science of material bodies puts him at odds with the traditional notion that the objects of geometrical investigation are radically distinct from the realm of material things. The difference becomes clearer when we examine the Hobbesian treatment of first principles, and more specifically the definitions upon which the science of geometry is based. True to his materialistic principles, Hobbes demands not only that all geometric objects be defined as bodies, but also that proper definitions must identify the causes by which such objects are produced, with the further requirement that the only such causes can be motions. The result is a conception of geometry that diverges significantly from the tradition.
Euclid defines the point as “that which has no part,” while a line is defined as “breadthless length” (Elements, Book I, Defs. 1, 2). Hobbes argues in De Principiis et Ratiocinatione Geometrarum (hereafter PRG) that these definitions are ambiguous:
these words “which has no part”, can be understood in two ways, either for undivided (a part indeed not being understood except where a division has gone before) or for indivisible, because it is by its nature incapable of division. In the former sense a point is rightly called a quantity, in the latter not; as all quantity is always divisible into divisibles. And thus if a point is indivisible, every line will lack breadth, and because there is nothing long that does not have breadth, the line would clearly be nothing. Although length is indeed not broad, nevertheless everything long is broad. It seems that Euclid himself was also of this opinion, that although a point has no parts actually, it is nevertheless divisible potentially and is a quantity, otherwise he would not have postulated that a straight line can be drawn from a point to a point.5 Which is impossible unless the line has some breadth.
(OL IV.391)