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Next: 3.2 Descartes and Leibniz Up: 3. Problems in Classical Previous: 3. Problems in Classical


3.1 Newtonian Physics

The idea most people have of physical substances is one that derives from Democritus, and was systematised by Boyle and Newton in the seventeenth century. This is that the world is ultimately made up of small solid `atoms' or `corpuscles.' These corpuscles have a definite position and size, and within their volume are solid and impenetrable. They are the ultimate atoms, since they cannot be broken up. They can fly around in space, and bounce off each other perfectly elastically. It was claimed that all phenomena in nature could be explained in terms of the movements and collisions of the corpuscles. It would have been nice if nature had been so simple.   Newton soon realised that his gravitational attraction could not be explained purely in terms of movements and collisions of corpuscles, much though he may have desired or imagined such an explanation.

  John Locke was one philosopher in Newton's time who took the ideas of nature and dispositional properties seriously.   He thought that the ultimate corpuscles ought to have `real essences' which from which all their potentialities and changes could be derived. He defined real essences as `the internal, but generally (in substances) unknown, constitution, whereon their discoverable qualities depend', but saw that the sciences of his day were a long way from discovering them. We see now that those sciences not only lacked means of investigation, but also the theoretical concepts needed to explain the observed phenomena: they could not even begin to explain such an elementary property of bodies as their cohesive strength.

  Locke saw that the cohesive forces that hold any object together cannot be explained in terms of movements and collisions of corpuscles (unless, following the Greeks, atoms were to have little hooks to hold on to each other). In his time, it was sometimes imagined that the cohesion of several parts of matter could be explained by the pressure of the particles of air on the exterior of the parts. It is difficult, for example, to separate two smooth panes of glass, because of the atmospheric pressure acting on the outside of the panes. Locke, however,3.1 points out the empirical difficulties and logical fallacies in these ideas. They are empirically inadequate, he argues, because an external pressure can never oppose either lateral or torsional forces, only longitudinal stretching. The ideas are logically inadequate too:

For though pressure of the particles of air may account for cohesion in matter that is grosser than air, and have pores less than the corpuscles of air, yet the weight or pressure of air will not explain, nor can be a cause of, the coherence of the particles of air themselves. And if the pressure of the aether, or any subtiler matter than air, may unite and hold fast together the parts of a particle of air, as well as other bodies, yet it cannot make bonds for itself and hold together the parts that make up the very least corpuscle of that materia subtilis3.2

He that could find the bonds that tie these heaps of loose little bodies together so firmly, he that could make known the cement that makes them stick so fast to one another, would discover a great and yet unknown secret; and yet when that was done, would he be far from making the extension of the body (which is the cohesion of its solid parts) intelligible, till he could show wherein consisted the union or the consolidation of the parts of those bonds or of that cement or of the least particle of matter that exists? Whereby it appears that this primary and supposed obvious quality of body will be found, when examined, to be as incomprehensible as anything belonging to our minds.3.3

    Boscovich accepted Newton's idea of gravity acting at a distance, and generalised it to some complicated dependence on distance of the force acting between pairs of particles. The forces could now be either attractive or repulsive, or both, at different distances. At large distances there would be Newton's gravitational attraction, at small distances there would be a larger attractive force responsible for the cohesion of solids and chemical compounds, and at the smallest distances a repulsive `core' is operative. In their physics, if not in their philosophy, both Newton and Boscovich took `forces' to be the real manner of operation between corpuscles. Boscovich took the `force field' at a place to describe how a test charge would react if placed there. Because forces therefore describe capacities to cause acceleration, and not the accelerations themselves, they are inherently dispositional, and can be used to explain observable dispositional properties such as, for example, the solubility of salt or the hardness of steel.   The standard of natural philosophy, for Hemholtz for example, became to describe inter-particle forces whose magnitude only depends on the distance between the particles.     As Dijksterhuis [1961, p. 490] remarks, `if Huygens could have read such formulations, it would have filled him with endless astonishment that this was called mechanistic science. However, from the moment Newton's way of thinking began to set its mark on physics, no concept had become more indissolubly bound up with the mechanistic view of nature than that of the force acting at a distance and causing motion.'    

      Later scientists soon realised that the electric and magnetic effects they discovered had no simple explanation in terms of movements and collisions, or in terms of radial forces. Faraday found that his best explanation was in terms of electric and magnetic fields, and, as made clear in Maxwell's theory of electromagnetism, these fields could travel around in space as if they were separate substances. But what then is a field? If they are not fluctuations in an aether, are they related to Newton's corpuscles, or are they some new kind of physical particle?

In the corpuscular theory there are in general severe philosophical problems when it comes to seeing the exact relation between the corpuscles and their dispositional forces of interaction (whether gravitational, electric, magnetic, or other). This is because the corpuscles were assumed to be purely actual and definite in every respect, and hence could never from their own nature have any such thing as a disposition, a potential field or a force for interactions. For potentials and forces are dispositional properties which may or may not operate. Remember from physics that a field potential at a place describes how a test charge would react if placed there.   According to the original ideas of corpuscular theory (e.g. of Huygens), this uncertainty means that potentials are not sufficiently actual and definite to be given to corpuscles.

These problems show that we will have to go back again to philosophical considerations about what kinds of substances there may possibly be.   In our philosophy of nature, we ought to reconsider those general principles on which we based our very idea of atomic corpuscles. Let us look at the alternatives that were considered in Newton's time.

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Next: 3.2 Descartes and Leibniz Up: 3. Problems in Classical Previous: 3. Problems in Classical
Prof Ian Thompson


Author: I.J. Thompson (except as stated)