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Libertus Fromondus (Libert Froidmont) (3 September 1587 – 28 October 1653) was, from 1634, the professor of Scripture at Louvain, the position he inherited from Jansenius, who also left to his care the publication of Augustinus. He studied and taught philosophy and obtained a doctorate in Theology in 1628. He kept a traditionalist Aristotelian view which is apparent throughout his objections to Descartes’ Discourse and Essays. He write a number of scientific books (Coenae saturnalitiae, variatiae Somnio sive Peregrinatione coelesti (Louvain, 1616); Dissertatio de cometa anni 1618 (Anvers, 1619); Meteorologicum libri VI (Anvers, 1627). Labyrinthus sive de compositione continui (Anvers, 1631) Commentarii in libros Quaestionum naturalium Senecae (Anvers, 1632); Anti-Aristarchus sive orbis terrae immobilis adversus Philippum Lansbergium (Anvers, 1634)), especially on meteorological problems, this being the reason why Descartes sent him his writings.

The exchange between Fromondus and Descartes, intermediate by Plempius, is the first and a very powerful example of the conflict between orthodox Aristotelianism and Cartesianism, or even between pre-modern and modern paradigms. Fromondus sent his 18 objections to Descartes’ Discourse and Essays on 13 September 1637 and Descartes responded to him on 3 October 1637. Out of these 18 objections, 3 are to the Discourse (the problem of mechanical and animal perception and the movements of the heart), 6 to Dioptrics (the nature of light and the process of seeing), and 9 to Meteors (the nature and composition of matter, especially water). Descartes responds punctually to each objection, not emphasising the disastrous consequences of his view for Aristotelianism, but praising his own mechanical method, which was despised by his opponent.

The main issue of Fromondus’ objections is Descartes’ ontology, i.e. what are the basic constituents of the world and how it functions. Descartes presupposes that the entire physical world is composed of particles of matter in motion that follow mechanical rules. Fromondus objects, in an orthodox Aristotelian way, that there has to be intentional species to account for the qualities of external objects and also for these objects to be perceived and known. He rejects Descartes’ probably most metaphysical statement that animals do not perceive, the composition of matter out of particles and the overall mechanical explanations of every phenomena.

1. Fromondus objects that perception cannot be accounted for only by heat because Descartes affirmed that all animal action can be explained by the heat that moves the particles. Or Descartes responds that actually animal do not see, feel, hear, smell. They are only machines and, for example, the images, imprinted mechanically by light on their retinas, mechanically cause their limbs to move.

2. Fromondus objects that if we allow animals to be similar to man-made machines then they would not need a substantial soul. But if that is true, maybe men do not need it either. Descartes responds by quoting Bible passages that animal souls are nothing else than their blood and that animals are really machines, but this, contrary to Fromondus’ concerns, prove better the nobility of human soul, its uniqueness and the superiority of human rational knowledge.

3. Fromondus objects that it is not possible for the blood to dilate sufficiently in the heart unless the temperature is that of a furnace. Descartes responds that the blood in veins and arteries is always near to the boiling point and in the heart it easily evaporates with only little more heat.

4. Fromondus objects that light is not a rod that instantaneously presses the eye and that light is not a subtle matter that passes through pores because it passes through glass which have no pores. Descartes replies that he does not presupposes vacuum trough which the rays to pass but only a subtle matter that press even through glass’ pores. The argument of Fromondus, with the sound, is rejected because sound is diminished even by curtains that definitely have pores. Fromondus also objects here that, according to Descartes, every (local) movement is light.

5. Fromondus affirms that intentional species, that he identify with the retinal images, are necessary for vision. Descartes respond first to the previous objection that rapid movement produce heat, and high heat is luminous, therefore every movement is in certain circumstances light. Regarding intentional species (the basis of perception and knowledge for Aristotelians) he merely ignores the objection.

6. Fromondus objects that refraction is not sufficiently explained. Descartes, in response, gives only the geometrical reason for the ray path.

7. Fromondus objects to Descartes’ claim that water is more permissive for the light than air. Descartes restate the claim. (In fact, Descartes’s sinus law for refraction i/n=sin i/sin n is incorrect because his conviction that light travels more rapidly in water than in air; the actual law is i/n=sin n/sin i, where i, n – speed of light in medium, respectively the incident angles)

8. Fromondus objects again that perception cannot be accounted for only by particles in motion. How the colours and their various properties can be accounted for only by the force of the incident ray? Descartes suggests him to read the demonstrations in Dioptrics.

9. Fromondus objects that sensations cannot be only in brain because we feel their qualities in our body. Descartes replies with the example of phantom limbs.

10. Fromondus objects than bodies cannot be composed of particles linked together because they are continuums. Descartes responds by defending his mechanical way of doing philosophy, as truer than any other, he rejects the obscure notions of Schools’ philosophy and Fromondus’ book on continuum and give examples like stones, wood and meat that are composed of some really distinct parts.

11. Fromondus objects that water cannot be composed from parts oblong like eels. Descartes replies that he overdemonstrated that and gives some “demonstrations” in the form of Scholastic probable syllogism.

12. Fromondus objects that the type of local movement cannot produce the various sensations and that real qualities are necessary. Descartes replies ironically, comparing different types of rubbing hands.

13. Fromondus affirms that in fact coldness does not rarefy the water. Descartes explain the phenomenon by his eel hypothesis.

14. Fromondus objects to Descartes’ explanation that dust is thrown into the air by sun rays. Descartes replies that we can feel the sun rays and that the air is not vacuum and it can sustain dust particles.

15. Fromondus affirms that the explanation of water smooth surface must be deduced from Archimedes’ laws of equilibrium. Descartes responds that those laws cannot account for smoothness.

16. Fromondus affirms that the cause of rarefaction is not the local movement. Descartes replies that a body rarefies if his particles diverge one from the other and the space is filled with other particles. Here Fromondus and Descartes talk past each other.

17. Fromondus claims that if water is salty only because the pointy particles press on the tongue, water would taste differently if these particles will press with different part. And he is generally dissatisfied with Descartes’ explanations that use only position and local motion. Descartes affirms that if water press with the smooth parts it does not elicit any taste. And he claims that his mechanical theory explains much more than any other theory and thus it is better, even if Fromondus calls it stupid, gross, ignoble and brute.

18. Fromondus objects that winds cannot be explained by a compression similar to that in an eolipile. Descartes affirms that he gave other reasons as well but that the compression theory is valid according to mechanical principles because the compression is not as great as in an eolipile.