Lenses and Waves

1690 - TRAITÉ DE LA LUMIÈRE 241
paper contained a mechanistic theory that remained essentially qualitative. In
the light of the precepts of Traité de la Lumière it could not be called an
instance of ‘true philosophy’.
The message of the paper could not, however, be mistaken. It made clear
what, in Huygens’ view, mechanistic explanation ought to be about. His was
the fourth and last of a series of papers on gravity read at the Académie in
August 1669. On the 7th, Roberval had opened the debate with a paper that
seems to be an express denial of everything Huygens stood for. 93 He chose to
account for gravity by proposing a mutual attraction between bodies of the
same kind. He did specify how such an attraction explained the properties of
gravity. He explicitly rejected efforts to explain it by the movement of a
subtle matter because he had never seen anything that was not problematic.
In his reaction to Huygens’ paper, four weeks later, Roberval said:
“… he excludes from nature without proof attractive and expulsive qualities and he
wants to introduce without foundation solely sizes, shapes and movement.” 94
In reply, Huygens said that he excluded those qualities because
“… I search for an intelligible cause of gravity, as it seems to me that it would be saying
as much as nothing when attributing the cause why heavy bodies descend to the earth
to some attractive quality of the earth or of these bodies themselves, but for the
movement, the shape and the sizes of bodies I do not see how one can say that I
introduce them without foundation since the senses make use know that these things
are in nature.” 95
This, then, was the raison d’être of the paper on gravity. In Roberval’s use of
attractive qualities he saw a relapse into the incomprehensible thinking
Descartes had dispensed with. He would not accept active principles in
nature that were not intelligible in terms of matter in motion.
But did Huygens regard attractive qualities as incomprehensible only
because of his Cartesian leanings? I think not. The 1669 paper on gravity had
been preceded by years of mathematical study of motions. In these he had
persistently reduced all phenomena of impact and acceleration to the
Galilean science of motion. Westfall describes in full detail his “… constant
effort to eliminate dynamic concepts and to treat mechanics as kinematics,
…” 96 Huygens found that whatever could not be expressed in terms of
velocities escapes mathematical treatment. In my view, Huygens rejected
attractive forces not only because his mechanistic convictions forbade them,
but also because such concepts were problematic from a mathematical point
93 OC19, 628-630. Frenicle and Buot followed on 14 and 21 August respectively, summarised in OC19,
630-631. Huygens read his paper – the longest – on 28 August. On 4 September, Roberval and Mariotte
gave their comments to which Huygens replied on 23 October; OC19, 640-644.
94 OC19, 640. “… il exclud de la nature sans preuve les qualitez attractives et expulsives et il veut
introduire sans fondement les seules grandeurs, les figures et le mouvement.”
95 OC19, 642. “… parce que je cherche une cause intelligible de la pesanteur, car il me semble que ce seroit
dire autant que rien que d’attribuer la cause pourquoy les corps pesants descendent vers la terre, a quelque
qualité attractive de la terre ou des corps mesmes, mais pour le mouvement la figure et les grandeurs des
corps je ne vois pas comment on peut dire que je les introduicts sans fondement puisque les sens nous
font connoistre que ces choses sont dans la nature.”
96 Westfall, Force, 177.