Rene Descartes - 'Principles of Philosophy' and 'Treatise on Light'

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Like Newton and Gilbert, Descartes published his major works in Latin - but he did oversee approved translations into his native French - including his Principia Philosophiae (Principles of Philosophy) which had first been published in 1644. Most of his published works were philosophical but did include some acclaimed 'science'.

Below we will look at some of his 'Principles of Philosophy' and 'Treatise on Light', and his 'The World' published after his death and giving more on his physics is dealt with in a separate section. An English translation of part of Descartes' Principia Philosophiae (Principles of Philosophy), though not approved by him, can be read online at http://www.classicallibrary.org/descartes/principles/ - a good website.

Descartes' Principles of Philosophy translated by John Veitch.

You can read in our section on Descartes 'The World', his argument concluding that the only essential property of a body is extension or space-occupancy, and that all extension assumed some body and so there could not be any empty space only dead matter and the universe must be infinite. He also argues that the only other certain property of a body is motion, and that 'motion can only be produced by other motion' - only by pushings or pullings.

So in Part 4, Descartes argues that "we can perceive no external objects unless some local motion be caused by them in our nerves, and that such motion cannot be caused by the fixed stars, owing to their great distance from us, unless a motion be also produced in them and in the whole heavens lying between them and us (through a continuous material ether)" Descartes like Aristotle basically opposed empty space on theoretical grounds.

(Gilbert saw motion as only a derivative of primary forces associated with bodies, like magnetism and gravity, and saw empty space as generating no forces and so being really empty. Earth's atmosphere attenuating with altitude and planet orbits not suffering drag were seen as evidence for empty space. Newton concluded that the chief property of matter is inertia and any piece of space generating no resistance to motion must be empty space - and perhaps came near to adding a second chief property of matter as gravity implying that any piece of space generating no gravity must be empty space containing no matter ? Of course Newton taking a black-box position did not actually conclude the latter, and allowed there might be some massless bodies with no gravity.)

In his Principles, Descartes repeatedly argued against empty space, to make a material ether central to his physics, hence ;

"XVIII. How the prejudice of an absolute vacuum is to be corrected.
.....And accordingly, if it be asked what would happen were God to remove from a vessel all the body contained in it, without permitting another body to occupy its place, the answer must be that the sides of the vessel would thus come into proximity with each other.
For two bodies must touch each other when there is nothing between them, and it is manifestly contradictory for two bodies to be apart, in other words, that there should be a distance between them, and this distance yet be nothing; for all distance is a mode of extension, and cannot therefore exist without an extended substance."

With empty space logically abolished, Descartes' imagining fills his universe with three types of matter, or elements. The first element is matter made up of a non-particle fluid moving so quickly that it shatters any body it hits and produces heat and light, and the sun and stars are composed of this element. The matter of the second element is made up of microscopic spherical particles, making a stable fluid, and this element fills space and propagates light. Finally, the third element of which planets and common objects are formed is made of larger particles least well-suited to motion.

He tackles light as the particles of his second element transmitting motions, somehow in a straight line instantly, "like a stick transmits a push on one end to the other end" - though for sound he used a normal wave theory. And to explain magnetism Descartes claimed that novel emitted effluvia particles of "threaded parts" passed through a network of one-way threaded passages in iron and worked like a corkscrew.

Interestingly Descartes considered light at some length as a signal ;

Descartes' Treatise on Light - translated by George MacDonald Ross

"Chapter I: On the difference between our sensations and the things that produce them

In proposing to write this treatise on Light, the first thing I want to bring to your attention is the fact that there can be a difference between the sensation we have of it (that is, the idea of it formed in our imagination via our eyes), and what there is in the objects which produce this sensation in us (that is what there is in flame or the sun which is called 'light'). For although most people are convinced that the ideas we have in our thinking are entirely similar to the objects they come from, I can see absolutely no reason why we should be certain of this - on the contrary, I am aware of many observations which should make us doubt it.

You know, of course, that words make us form conceptions of the things they signify even though they have no resemblance to them, often even without our paying any attention to the sounds of the words or the syllables of which they are composed. Thus it can happen that, after hearing something said of which we have perfectly understood the sense, we are unable to say what language it was spoken in. But if words, which have meaning only as a human institution, are enough to make us form conceptions of things they bear no resemblance to, why could not Nature too have instituted some sign which would make us have the sensation of light, but without containing in itself anything similar to this sensation? And is this not how she has instituted smiles and tears to make us read joy and sadness on people's faces ?

But perhaps you will say that our ears really only make us perceive the sound of the words, and our eyes the countenance of the person who smiles or weeps, and that it is our spirit which, having grasped the meanings of the words and countenance, represents them to us at the same time. I could reply to this that it is likewise our spirit which represents to us the idea of light whenever the action which signifies it comes into contact with our eyes. But rather than wasting time in disputation, I would prefer to give another example.

When we ignore the meanings of words and listen only to their sound, do you think the idea of this sound formed in our thinking bears some resemblance to the object that causes it? Someone opens their mouth, moves their tongue, emits their breath - but I see nothing in all these actions that is not very different from the idea of the sound which they make us form in our imagination. And the majority of philosophers assure us that sound is nothing but a certain vibration of the air which comes and beats against our ears; so that if the sense of hearing brought the true image of its object into our thinking, instead of making us have a conception of sound, it would have to make us have a conception of the motion of the parts of the air then vibrating against our ears. But since not everyone, perhaps, will be prepared to believe what philosophers say, I shall give yet another example.

The sense which is considered the least deceptive and the most certain is that of touch; so, if I show you that even the sense of touch makes us conceive many ideas which have no resemblance at all to the objects that produce them, I do not think you should find it strange if I say that the sense of sight can do the same. There is no one who does not know that the ideas of tickling and of pain which are formed in our thinking on the occasion of our coming into contact with external bodies bear no resemblance to them. You gently pass a feather over the lips of a sleeping child, and it senses that you are tickling it: do you think that the idea of tickling which it conceives resembles in any respect the qualities of the feather? A soldier returns from a battle: during the heat of the action he could have been wounded without noticing it; but now that he is beginning to cool off, he feels some pain, and believes he has been wounded. A surgeon is called, his armour is removed, the surgeon makes a visit, and finally it is found that what he felt was nothing other than a buckle or a strap which had got caught up under his armour and caused the trouble by pressing into him. If his sense of touch, in making him aware of this strap, had impressed the image of it on his thinking, he would have had no need of the surgeon to tell him what he was feeling.

So, I see no reason why we should believe that whatever it is in objects that gives rise to our sensation of light is any more like that sensation than the actions of a feather or a strap are like the sensation of tickling or pain. However, I have certainly not brought up these examples in order to make you believe absolutely that light is different in objects from what it is in our eyes; but only in order to make you reserve judgment about it; and, by keeping you from being prejudiced by the contrary opinion, to enable you to join me now in a more fruitful examination of its nature. . . . ."

Most of Descartes actual science theory being basically 'logical imaginings', based on a weaker knowledge of actual physical phenomena than Gilbert or Newton, perhaps added little of practical use to physics theory at the time, with the exception of his work on light based on a particle theory and adding to knowledge on refraction especially. Newton published a strong disproof of Descartes' material ether and specifically of his ether vortex theory of planetary motion. Descartes had basically better organised ancient greek Atomist theory and incorporated it into his God-based Dualist philosophy so it could better suit religion. Descartes' physics gained wide support, and later Maxwell and Einstein produced alternative 'non-material' ether/continuum Descartes-style theories though they perhaps lacked the relatively clear simple logic of Descartes' material push ether physics.

Though modern science is really still based on Descartes-style dead matter mechanical push-physics theory, many of its statements in fact read like excited active matter theory statements :-
1. A typical modern explanation of part of Brain action - "A neuron accepts signals from other neurons through branchlike structures called dendrites. Whenever enough messages arrive from neighbouring neurons to excite it, a neuron sends an electrical impulse."
2. A typical modern explanation of part of Atomic action - "By absorbing photons of some one wavelength, an atom can be excited to any of various discrete energy levels and then it can emit light of various wavelengths."

These clearly read like active-neurons and active-atoms statements, and not like Descartes mechanical push statements. Even modern declared dead-matter theorists seem often to use active-matter language (as easier or clearer assumedly). From radioactivity and other atomic behaviour, we now KNOW that atoms are not simple small billiard balls as might best suit Descartes-style dead-matter theory - and often at least equally well fit an active-atoms theory akin to Gilbert's.

It was certainly Descartes' advances in mathematics that were of more practical use to progress in physics theory, and the same can probably be said of Newton and then of Einstein also ?

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