Chapter II: Part 2

[118] M: p. 92 (see also p. 339). Although Gilbert does not
make it explicit, this would solve the medieval problem of
gravitation without resorting to a Ptolemaic universe. In
addition, since coacervation is electric, and electric forces
can be screened, it should have been possible to reduce the
downward motion of a body by screening!

The analogy between electric attraction and fluids is a most concrete one, yet lying beneath this image is a hypothesis that is difficult to fix into a mechanical system based upon contact forces. This is the assumption that under the proper conditions bodies tend to move together in order to participate in a more complete unity.[119] The steps in electrical attraction were described as occurring on two different levels of abstraction: first one has physical contact through an effluvium or "spiritus" that connects the two objects physically. Then, as a result of this contact, the objects somehow sense[120] that a more intimate harmony is possible, and move accordingly. Gilbert called the motion that followed contact, attraction. However, this motion did not connote what we would call a force:[121] it did not correspond directly to a push or pull, but it followed from what one might term the apprehension of the possibility of a more complete participation in a formal unity. The physical unity due to the "spiritus" was the prelude to a formal organic unity, so that _humor_ is "rerum omnium unitore." Gilbert's position can be best seen in the following:[122]

Spiritus igitur egrediens ex corpora, quod ab humore aut
succo aqueo concreverat, corpus attrahendum attingit,
attactum attrahenti unitur; corpus peculiari effluviorum
radio continguum, unum effecit ex duobus: unita confluunt in
conjunctissimam convenientiam, quae attractio vulgo dicitur.
Quae unitas iuxta Pythagorae opinionem rerum omnium
principium est, per cuius participationem unaquaeque res una
dicitur. Quoniam enim nullo actio a materia potest nisi per
contactum, electrica haec non videntur tangere, sed ut
necesse erat demittitur aliquid ab uno ad aliud, quod proxime
tangat, et eius incitationis principium sit. Corpora omnia
uniuntur & quasi ferruminantur quodammodo humore ...
Electrica vero effi via peculiaria, quae humoris fusi
subtilissima sunt materia, corpuscula allectant. Aer (commune
effluvium telluris) & partes disjunctis unit, & tellus
mediante aere ad se revocat corpora; aliter quae in
superioribus locis essent corpora, terram non ita avide
appelerent.

Electrica effluvia ab aere multum differunt, & u aer telluris
effluvium est, ita electrica suahabent effluvia & propria;
peculiaribus effluviis suus cuique; est singularis ad
unitatem ductus, motus ad principium, fontem, & corpus
effluvia emittens.

A similar hypothesis will reappear in his explanation of magnetic attraction.

[119] M: pp. 91, 92: "This unity is, according to Pythagoras,
the principle, through participation, in which a thing is
said to be one" (see footnotes 30 and 122).

[120] "Sense" is probably too strong a term, and yet the
change following contact is difficult to describe in
Gilbert's phraseology without some such subjective term. See
Gilbert's argument on the soul and organs of a loadstone, M:
pp. 309-313.

[121] M: pp. 112, 113.

[122] Gilbert, _De magnete_, London, 1600, bk. 2, ch. 2, pp.
56-57.

Following the tradition of the medieval schoolmen Gilbert started his examination of the nature of the loadstone by pointing out the different kinds of motion due to a magnet. The five kinds (other than up and down) are:[123]

(1) coitio (vulgo attractio, dicta) ad unitatem magneticam
incitatio.

(2) directio in polos telluris, et telluris in mundi
destinatos terminos verticitas et consistentia.

(3) variatio, a meridiano deflexio, quem motum nos depravatum
dicimus.

(4) declinatio, infra horizontem poli magnetici descensus.

(5) motus circularis, seu revolutio.

Of the five he initially listed, three are not basic ones. Variation and declination he later explained as due to irregularities of the surface of the earth, while direction or verticity is the ordering motion that precedes coition.[124] This leaves only coition and revolution as the basic motions. How these followed from "the congregant nature of the loadstone can be seen when the effusion of forms has been considered."

Coition (he did not take up revolution at this point) differed from that due to other attractions. There are two and only two kinds of bodies that can attract: electric and magnetic.[125] Gilbert refined his position further by arguing that one does not even have magnetic attraction[126] but instead the mutual motion to union that he called coition.[127] In electric attraction, one has an action-passion relation of cause and effect with an external agent and a passive recipient; while in magnetic coition, both bodies act and are acted upon, and both move together.[128] Instead of an agent and a patient in coition,[129] one has "conactus." Coition, as the Latin origin of the term denoted, is always a concerted action. [130] This can be seen from the motions of two loadstones floating on water.[131] The mutual motion in coition was one of the reasons for Gilbert's rejection of the perpetual motion machine of Peregrinus.[132]

[123] _Ibid._, ch. 1, pp. 45-46.

[124] M: pp. 110, 314.

[125] M: pp. 82, 105, 170, 172, 217.

[126] M: p. 98.

[127] M: pp. 100, 112, 113, 143, 148. It need hardly be
pointed out that coitus is not an impersonal term.

[128] M: p. 110.

[129] M: p. 110.

[130] M: pp. 109, 115, 148, 149, 155, 166, 174.

[131] M: pp. 110, 155.

[132] M: pp. 166, 332. See also footnote 84.

Magnetic coition, unlike electric attraction, cannot be screened.[133] Hence it cannot be corporeal for it travels freely through bodies[134] and especially magnetic bodies;[135] one can understand the action of the armature on this basis.[136] Since coition cannot be prevented by shielding, it must have an immaterial cause.[137]

[133] M: pp. 90, 106, 107, 108, 113, 132, 135, 136, 158. This
is, of course, contrary to modern experience.

[134] M: pp. 106, 107, 108, 114, 134, 136, 140, 162.

[135] M: pp. 106, 109, 114, 159, 162.

[136] M: pp. 137-140.

[137] M: p. 109.

Yet, unless one has the occult action-at-a-distance, change must be caused by contact forces. Gilbert resolved the paradox of combining contact forces with forces that cannot be shielded, by passing to a higher level of abstraction for the explanation of magnetic phenomena: he saw the contact as that of a form with matter.

Although Gilbert remarked that the cause of magnetic phenomena did not fall within any of the categories of the formal causes of the Aristotelians, he did not renounce for this reason the medieval tradition. Actually there are many similarities between Gilbert's explanation of the loadstone's powers and that of St. Thomas. Magnetic coition is not due to any of the generic or specific forms of the Aristotelian elements, nor is it due to the primary qualities of any of their elements, nor is it due to the celestial "generans" of terrestrial change.[138]

Relictis aliorum opinionibus de magnetis attractione; nunc
coitionis illius rationem, et motus illius commoventem
naturam docebimus. Cum vero duo sint corporum genera, quae
manifestis sensibus nostris motionibus corpora allicere
videntur, Electrica et Magnetica; Electrica naturalibus ab
humore effluviis; Magnetica formalibus efficientiis, seu
potius primariis vigoribus, incitationes faciunt. Forma ilia
singularis est, et peculiaris, non Peripateticorum causa
formalis, et specifica in mixtis, est secunda forma, non
generantium corporum propagatrix; sed primorum et praeciporum
globorum forma; et partium eorum homogenearum, non
corruptarum, propria entitas et existentia, quam nos
primariam, et radicalem, et astream appellare possumus
formam; non formam primam Aristotelis; sed singularem illam,
quae globum suum proprium tuetur et disponit. Talis in
singulis globis, Sole, lunas et astris, est una; in terra
etiam una, quae vera est ilia potentia magnetica, quam nos
primarium vigorem appellamus. Quare magnetica natura est
telluris propria, eiusque omnibus verioribus partibus,
primaria et stupenda ratione, insita; haec nec a caelo toto
derivatur procreaturve, per sympathiam, per influentiam, aut
occultiores qualitates; nec peculiari aliquo astro: est enim
suus in tellure magneticus vigor, sicut in sole et luna suae
formae; frustulumque; lunae, lunatice ad eius terminos, et
formam componit se; solarque; ad solem, sicut magnes ad
tellurem, et ad alterum magnetem, secundum naturam sese
inclinando et alliciendo. Differendum igitur de tellure quae
magnetica, et magnes; tum etiam de partibus eius verioribus,
quae magneticae sunt; et quomodo ex coitione difficiuntur.

Instead, he declared it to be due to a form that is natural and proper to that element that he made the primary component of the earth.[139]

To understand his argument, let us briefly recall the peripatetic theory of the elements. In this philosophy of nature each element or simple body is a combination of a pair of the four primary qualities that informs inchoate matter. These qualities are the instruments of the elemental forms and determine the properties of the element. Thus the element fire is a compound of the qualities hot and dry, and the substantial form of fire acts through these qualities. Similarly for the other elements, earth, water, and air: their forms determine a proper place for each element, and a motion to that place natural to each element.[140]

[138] M: p. 105, and Gilbert, _De magnete_, London, 1600, bk.
2 ch. 4, p. 65.

[139] M: p. 105.

[140] M: pp. 289, 322.

Gilbert had previously declared that the primary substance of the earth is an element. Since it is an element, it has a motion natural to it, and this motion is magnetic coition. As an Aristotelian considered the substantial form of the element, fire, to act through the qualities of hot and dry, and to cause an upward motion; so Gilbert argued that the substantial form of his element, pure loadstone, acts through the magnetic qualities and causes magnetic coition. This motion is due to its primary form, and is natural to the element earth.[141] It is instilled in all proper and undegenerate parts of the earth,[142] but in no other element.[143]

[141] M: pp. 26, 68, 105, 179, 198, 307, 335, 343. For
rotation, see footnote 147.

[142] M: pp. 67, 71. That each part is informed with the
properties of the whole is an argument favoring an animistic
explanation of the nature of this form.

[143] M: p. 109.

To the medieval philosopher, the "generantia" of the occult powers of the loadstone are the heavenly bodies. Gilbert, however, endowed the earth with these heavenly powers which were placed in the earth in the beginning[144] and caused all magnetic materials to conform with it both physically and formally.[145] Such magnetic powers are the property of all parts of the earth;[146] they give the earth its rotating motion[147] and hold the earth together in spite of this motion.[148]

[144] M: pp. 111, 188.

[145] M: pp. 67, 105, 179, 183.

[146] M: pp. 101, 105, 217.

[147] M: pp. 179, 304, 305, 311, 322, 326, 328, 330-334,
338-343.

[148] M: pp. 142, 179; see also electric attraction, p. 97.

Indeed, each of the main stellar bodies, sun, moon, stars, and earth, has such a form or principle unique to itself that causes its parts not only to conform with itself but to revolve.[149] Thus, if one removes a piece of the moon from this body, it will tend to align itself with the moon and then to return to its proper place; and a fragment of the sun would similarly tend to return after proper orientation.[150] Moreover, there is a farther-ranging, though weaker, mutual action of the heavenly bodies so that one has a causal hierarchy of these specific conforming powers. The form of the sun is superior to that of the inferior globes and is responsible for the order and regularity of planetary orbits.[151] In like manner, the moon is responsible for the tides of the ocean.[152]

[149] M: pp. 308, 317-343.

[150] M: pp. 106, 340.

[151] M: pp. 308, 309, 311, 330, 333, 344, 347.

[152] M: pp. 136, 334, 345.

By virtue of the causal hierarchy of forms, the loadstone acquires its magnetic powers from the earth.[153] As the earth has its natural parts, so has the stone.[154] Although the geometrical center of a terrella is the center of the magnetic forces,[155] objects do not tend to move to the center but to its poles,[156] where the magnetic energy is most conspicuous.[157] However, in a sense, the energy is everywhere equal: the virtue is spread throughout the entire mass of the loadstone,[158] and all the parts direct the forces to the poles.[159] The poles become the "thrones" of the magnetic powers.[160] On the other hand, the directive force is stronger where coition is weaker and accordingly, verticity is most prominent at the equator.[161]

[153] M: pp. 184-186, 190, 232. This is not quite the same
argument as that the powers of the loadstone are identical
with those of the earth. See footnote 78.

[154] M: pp. 125, 180.

[155] M: p. 151.

[156] M: pp. 121, 150.

[157] M: pp. 115, 151, 165.

[158] M: pp. 106, 118, 151, 191, 205, 221, 243.

[159] M: pp. 116, 117, 119, 131, 183, 188, 221.

[160] M: p. 31.

[161] M: pp. 116, 151, 200.

The strength of a loadstone depends upon its shape and mass. A bar magnet has greater powers than a spherical one because it tends to concentrate the magnetic powers more in the ends.[162] For a given purity and shape, the heavier the loadstone, the greater its strength.[163] A loadstone has a maximum degree of magnetic force that cannot be increased.[164] However, weaker ones can be strengthened by stronger ones.[165] Similarly, the shape and weight of the iron determine the magnetic force in coition.[166]

[162] M: pp. 131, 132, 153-158.

[163] M: pp. 141, 152, 153, 158, 161, 191, 222.

[164] M: p. 146.

[165] M: p. 165.

[166] M: p. 153.

The formal forces of a loadstone emanate in all directions from it,[167] but there is a bound to it that Gilbert called the "orbis virtutis."[168] The shape of this "orbis virtutis" is determined by the shape of the stone.[169] This insensible effusion is analogous to the spreading of light that reveals its presence only by opaque bodies.[170] Similarly, the magnetic forms are effused from the stone,[171] and can only reveal their presence by coition with another loadstone or by "awakening" magnetic bodies within the "orbis virtutis."[172] Unmagnetized iron that comes within the "orbis virtutis" is altered, and the magnetic virtue renews a form that is already potentially in the iron.[173] The formal energy is drawn not only from the stone but from the iron.[174] This is not generation, or alteration in the sense of a new impressed quality, but alteration in the sense of the entelechy or the activation of a form potentially present.[175] Those bodies magnetized by coming within the "orbis virtutis" have in turn an efflux of their own.[176] Iron can also receive verticity directly from the earth without the intervention of an ordinary loadstone.[177] Such verticity can be expelled and annulled by the presence of another loadstone.[178]

[167] M: pp. 121, 123, 124, 304, 305, 306, 307, 309.

[168] Gilbert defined the _orbis virtutis_ in the glossary at
the beginning of his treatise as, "... totum illud spatium,
per quod quaevis magnetis virtus extenditur." This is the
core of the difference between electric and magnetic forces.
The substantial form of an electric could not be "effused,"
but was "imprisoned" in matter (as the Neoplatonic soul in
the human body); while the primary form of a magnet did not
require a material carrier and its effusion was similar to
the propagation of a species in light.

[169] M: pp. 124, 150, 151.

[170] M: pp. 123, 307.

[171] M: pp. 304-307. See also p. 310, where it is stated
that the sun and earth could awaken souls.

[172] M: pp. 101, 110, 112, 123, 148, 149, 304, 305. This
awakening of the iron within the "orbis virtutis" is
comparable (pp. 216, 350) to the birth of a child under the
influence of the stars.

[173] M: pp. 110, 111, 112, 189, 216, 217. See also footnote
36.

[174] M: p. 106.

[175] M: pp. 106, 109, 110.

[176] M: pp. 113, 114.

[177] M: pp. 190, 192, 210-216.

[178] M: p. 209.

Although one does not normally find iron to be magnetized, a loadstone always has some magnetism. That two bodies such as iron and loadstone should have different properties is the result of the loss of a form by the iron, but this form is still potentially present in the iron. The iron that has been obtained from an ore has been deformed,[179] for it has been placed "outside its nature" by the fire.[180] The nature has not been removed, since, once the iron has cooled, the confused form can be reformed by a loadstone. [181] The latter "awakens" the proper form of iron.[182] After smelting, the magnetized iron may manifest stronger powers than a loadstone of equal weight, but this is because the primary matter of the earth is purer in the iron than in the loadstone.[183] If fire does not deform a loadstone too much, it can be remagnetized,[184] but a burnt loadstone cannot be reformed.[185] Corruption from external causes may also deform a loadstone or iron so that it can not be magnetized.[186] Bodies mixed with the degenerate substance of the earth or with aqueous humor spoilt by contamination with earth, do not show either electric attraction or magnetic coition.[187]

[179] M: pp. 107, 110, 111.

[180] M: p. 108.

[181] M: pp. 111, 112, 113.

[182] M: pp. 109, 111, 112, 148, 149.

[183] M: pp. 112, 149.

[184] M: pp. 142, 189.

[185] M: p. 190.

[186] M: pp. 85, 105, 113, 143, 226.

[187] M: p. 84.

In a manner suggestive of Peregrinus, Gilbert wrote that, "magnetic bodies seek formal unity."[188] Thus a dissected loadstone not only tends to come back together, as in the unordered coacervation of electric attraction, but to restore the organization it had before dissection.[189] Accordingly, opposite poles appear on the interfaces of the sections, not "from an opposition" but from "a concordance and a conformance."[190] This ensures that when the parts are joined together again, they have the same orientation as before. Gilbert compared this power of restoring the original loadstone with that of a plant's vital power under the process of cutting and grafting; the plant can be revived only when the parts are in a certain order.[191]

[188] M: p. 186.

[189] M: pp. 185-188. See also footnote 31.

[190] M: pp. 186, 193.

[191] M: pp. 199-200.

A hypothesis similar to that used to explain electric attraction lay beneath the explanation of magnetic coition: that bodies brought into contact will move together. In electric attraction, the contact is material and due to the "spiritus" from the electric body; in magnetic coition, it is formal and depends on the action of a primary form that spreads from a magnetized body to its limit of effusion, the "orbis virtutis." If iron is inside the "orbis virtutis," the two bodies "enter into alliance and are one and the same"[192] for within it "they have absolute continuity, and are joined by reason of their accordance, albeit the bodies themselves be separated."[193]

Gilbert's treatment of coition can be analyzed into the same two steps as can electric attraction. First occurs a contact, which in this case is not physical but formal, and from this initial formal contact follows movement to a more complete unity. Both the contact and the movement to unity are described on the same level of abstraction, instead of on two different levels as in electric attraction. Again one does not find any clear-cut concept of force as a push or pull,[194] but instead, a motion to a formal unity, this time a cooperative motion. The parts of a magnetic body are in greater harmony when they are assembled in a certain pattern and so they move accordingly.

[192] M. p. 111.

[193] M: p. 112.

[194] See, however, M: pp. 112, 113.

As to the nature of the primary form itself, Gilbert agreed with Thales that it is like a soul,[195] "for the power of self-movement seems to betoken a soul."[196] With Galen and St. Thomas he placed the form of the loadstone superior to that of inanimate matter.[197] In a sense, Gilbert even made it superior to organic matter, for it is incapable of error.[198] Like the soul, the primary form cannot be fragmented; when a loadstone is divided, one does not separate the poles but each part acquires its own poles and an equator.

[195] M: pp. 109, 312.

[196] M: p. 109.

[197] M: p. 309.

[198] M: pp. 311-312.

Like the soul, fire does not destroy it.[199] Like the soul of astral bodies, and of the earth itself, it produces complex but regular motions; the motion of two loadstones on water offers such an example.[200] Like the soul of a newborn child, whose nature depends on the configuration of the heavens, the properties in the newly awakened iron depend upon its position in the "orbis virtutis."[201]

Whence Gilbert declared:

... the earth's magnetic force and the animate form of the
globes, that are without senses, but without error ... exert
an unending action, quick, definite, constant, directive,
motive, imperant, harmonious through the whole mass of
matter; thereby are the generation and the ultimate decay of
all things on the superficies propagated.[202] The bodies of
the globes ... to the end that they might be in themselves,
and in their nature endure, had need of souls to be conjoined
to them, for else there were neither life, nor prime act, nor
movement, nor unition, nor order, nor coherence, nor
_conactus_, nor _sympathia_, nor any generation nor
alteration of seasons, and no propagation; but all were in
confusion....[203] Wherefore, not with reason, Thales ...
declares the loadstone to be animate, a part of the animate
mother earth and her beloved offspring.[204]

Gilbert ended book 5 of his treatise on the magnet with a persuasive plea for his magnetic philosophy of the cosmos, yet his conceptual scheme was not too successful an induction in the eyes of his contemporaries. In particular the man from whom the Royal Society took the inspiration for their motto, "Nullius in verba," did not value his magnetic philosophy very highly. Whether Francis Bacon was alluding to Gilbert when he expounded his parable of the spider and the ant[205] is not explicit, but he certainly had him in mind when he wrote of the Idols of the Cave and the Idols of the Theater.[206]

[199] M: p. 108.

[200] M: p. 110.

[201] M: p. 216.

[202] M: p. 311.

[203] M: pp. 310, 311.

[204] M: p. 312.

[205] Francis Bacon, _op. cit._ (footnote 42), vol. 1,
_Novum organum_, bk. 1, ch. 95, p. 306.

[206] _Ibid._, ch. 54 and ch. 64 (pp. 259 and 267).

Few of the subsequent experimenters and writers on magnetism turned to Gilbert's work to explain the effects they discussed. Although both his countrymen Sir Thomas Browne[207] and Robert Boyle[208] described a number of the experiments already described by Gilbert and even used phrases similar to his in describing them, they tended to ignore Gilbert and his explanation of them. Instead, both turned to an explanation based upon magnetic effluvia or corpuscles. The only direct continuation of Gilbert's _De magnete_ was the _Philosophia magnetica_ of Nicolaus Cabeus.[209] The latter sought to bring Gilbert's explanation of magnetism more directly into the fold of medieval substantial forms.

[207] Sir Thomas Browne, _Pseudodoxia epidemica_, ed. 3,
London, 1658, bk. 2, ch. 2, 3, 4.

[208] Robert Boyle, _Experiments and notes about the
mechanical production of magnetism_, London, 1676.

[209] Nicolaus Cabeaus, _Philosophia magnetica_, Ferarra,
1629.

However, Gilbert's efforts towards a magnetic philosophy did find approval in two of the men that made the seventeenth century scientific revolution. While Galileo Galilei[210] was critical of Gilbert's arguments as being unnecessarily loose, he nevertheless saw in them some support for the Copernican world-system. Johannes Kepler[211] found in Gilbert's explanation of the loadstone-earth a possible physical framework for his own investigations on planetary motions.

[210] Galileo Galilei, _Dialogue on the great world systems_,
in the translation of T. Salusbury, edited and corrected by
G. de Santillana, University of Chicago Press, 1953, pp.
409-423.

[211] Cassirer, _op. cit._ (footnote 3), vol. 1, p. 359-367.

Yet Galileo and Kepler had moved beyond Gilbert's world of intellectual experience. They were no longer concerned with determining the nature of material things in order to explain their qualities. Instead, they had passed into the realm of the mathematical relations of kinematics: quantitative law had replaced qualitative experience of cause and effect. Gilbert had some intimations of the former, but he was primarily concerned with explaining magnetism in terms of substance and attribute. He had to ascertain the nature of the loadstone and of the earth in order to explain their properties and their motions. He even went further and explained the nature of the form of the loadstone.

His method of determining the nature of a substance was a rather primitive one--it was not by a process of induction and deduction, nor by synthesis and analysis, nor by "resolutio" and "compositio," but by the use of analogies. He compared the natural history of metals and rocks with that of plants, and gave the two former the same kind of principle as the last. He determined the nature of the entity behind electric attraction by finding that such attractions could be screened, and hence it had to be corporeal. After comparing this "corporeal" attraction with that of the surface forces of a fluid, he concluded that the entity was a subtle fluid. He determined the nature of the entity behind magnetic coition by (incorrectly) finding that it cannot be screened, and hence the cause had to be a formal one. Since both stars and the loadstone can carry out regular motions, and stars had souls, the form of the loadstone had to be a soul. The method of analogy was used again in his comparison of the properties of a magnetized needle placed over a terrella with the properties of a compass placed over the earth, whence he concluded the earth to be a giant loadstone. Since the earth resembled the other celestial globes, it had to have, the circular inertia of these globes.[212] As for his magnetic experiments to show physically that the earth moved, and his unbridled speculations on the "animae" of the celestial globes, one is inclined to agree with Bacon's estimate of his magnetic philosophy.

One might consider Gilbert's book as a Renaissance recasting of Aristotle's _De caelo_ with the earth in the role of a heavenly body. So it might well be, for Gilbert was still concerned with distinguishing the nature of the heavenly body, earth, that caused the coitional and revolving motions, from those natures for which up and down, and coacervation were the natural motions. Because the natural motions were different, the natures had to be different, and these different natures led to a universe and a concept of space neither of which were Aristotelian. One no longer had a central reference point for absolute space; there was no "motor essentialis" focused upon the earth but one had only the mutual motion of the heavenly bodies. The natural distinction between heaven and earth was gone, for the earth was no longer an inert recipient but a source of wonder, and so the stage was set for the universe of Giordano Bruno.[213] The Aristotelian philosophy of nature was used to justify a new cosmology, but there was no break with the past such as one finds in Galileo and Kepler. Instead he followed the chimera of the world organism, as Paracelsus had, and of the world soul, as Bruno had. Consequently Gilbert's physiology did not enter into the main stream of science.

[212] Because the earth has the same nature as a celestial
globe, its revolution and circular inertia require no more
explanation than those of any other heavenly body.

[213] One wonders if Bruno might not have been another of the
stimuli for Gilbert. The latter's interest in magnetism began
shortly before Bruno visited England and lectured on his
interpretation of the Copernican theory.

Yet this is not to deny Gilbert's services to natural philosophy. Although not all of his experimental distinction between electric and magnetic forces has been retained, still, some of it has. His "orbis virtutis" was to become a field of force, and his class of electrics, insulators of electricity. His practice of arming a loadstone was to be of considerable importance in the period before the invention of the electromagnet. His limited recognition of the mutual nature of forces and their quantitative basis in mass was ultimately to appear in Newton's second and third laws of motion. In spite of the weaknesses of the method of analogy, Gilbert's experimental model of the terrella to interpret the earth's magnetism was as much a contribution to scientific method as to the theory of magnetism.

Consequently, in spite of an explanation of electricity and magnetism that one would be amused to find in a textbook today, we can still read his _De magnete_ with interest and profit. But more important than his scientific speculations, is the insight he can give us into a Renaissance philosophy of nature and its relation to medieval thought. One does not find in _De magnete_ a prototype of modern physical science in the same sense one can in the writings of Galileo and Kepler. Instead one finds here a full-fledged example of an earlier kind of science, and this is Gilbert's main value to the historian today.