[ Pobierz całość w formacie PDF ]
particular, the crucial second premiss of the causal argument, the completeness of
physics, would seem already to have been available to Leibniz. So doesn't this mean that
everything needed to appreciate the causal argument was already to hand in the second
half of the seventeenth century, long before the rise of twentieth-century materialism?
Well, it was but only on the assumption that Leibniz gives us the correct dynamics.
However, Leibniz's physical theories were quickly eclipsed by those of Newton, and this
then reopened the whole issue of the completeness of physics.
The central point here is that Newton allowed forces other than impact. Leibniz, along
with Descartes and all other pre-Newtonian proponents of the mechanical philosophy ,
took it as given that all physical action is by contact. They assumed that the only possible
cause of a change in a physical body's motion is the impact of another physical body. (Or
more precisely, as we are telling the story, Descartes supposed that the only possible non-
mental cause of physical change is impact, and Leibniz then argued that mental causes
other than impact are not possible either, if the conservation of momentum and energy
are to be respected.)
Newtonian mechanics changed the whole picture. This is because Newton did not take
impact as his basic model of dynamic action. Rather, his basic notion was that of an
impressed force. Rather than thinking of force as something inside a body which might
be transferred to other bodies in impact, as did all his contemporaries (and indeed most of
his successors for at least a century4 ), Newton thought of forces as disembodied entities,
acting on the affected body from outside. An impressed force consists in the action only,
and remains no longer in the body when the action is over . Moreover, impressed forces
are of different origins, as from percussion, from pressure, from centripetal force
(Newton 1966 [1686]: 2, definition IV). Gravity was the paradigm. True, the force of
gravity always arose from the presence of massive bodies, but it pervaded space, acting
on anything that might be there, so to speak, with a strength as specified by the inverse
square law.
Once disembodied gravity was allowed as a force distinct from the action of impact, then
there was no principled barrier to other similarly disembodied special forces, such as
chemical forces or magnetic forces or forces of cohesion (cf. Newton 1952 [1704]:
queries 29 31) or indeed vital and mental forces.
Nothing in classical Newtonian thinking rules out special mental forces. While Newton
has a general law about the effects of his forces (they cause proportional changes in the
velocities of the bodies they act on), there is no corresponding general principle about the
causes of such forces. True, gravity in particular is governed by the inverse square law,
which fixes gravitational forces as a function of the location of bodies with mass. But
there is no overarching principle dictating how forces in general arise. This opens up the
possibility that there may be sui generis mental forces, which would mean that
Newtonian physics, unlike Leibnizian physics, is not physically complete. Some physical
processes could have non-physical mental forces among their causal antecedents. (Some
readers may be feeling uneasy about the way in which the completeness of physics has
now turned into an issue about what forces exist. I shall address this issue at the end of
this section.)
The switch from a pure impact-based mechanical philosophy to the more liberal world of
Newtonian forces undermined Leibniz's argument for the completeness of physics.
Leibniz could hold that the principles governing the physical world leave no room for
mental acts to make a difference because he had a simple mechanical picture of the
physical world. Bodies preserve their motion in any given direction until they collide, and
then they obey the laws of impact. The Newtonian world of disembodied forces is far less
pristine, and gives no immediate reason to view physics as complete.
You might think that the conservation laws of Newtonian physics would themselves
place constraints on the generation of forces, in such a way as to restore the completeness
of physics. But this would be a somewhat anachronistic thought. Conservation laws did
not play a central role in Newtonian thinking at least not in that of Newton himself and
his immediate followers. True, Newton's mechanics does imply the conservation of
momentum. This falls straight out of his Third Law, which requires that action and
reaction are always equal. But it is a striking feature of Newtonian dynamics that there is
no corresponding law for energy.5
Of course, as we shall see in the next section, the principle of the conservation of kinetic
and potential energy in all physical processes did eventually become part of the
Newtonian tradition, and this does impose a general restriction on possible forces, a
restriction expressed by the requirement that all forces should be conservative . But this
came much later, in the middle of the nineteenth century, and so had no influence on the
range of possible forces admitted by seventeenth- or eighteenth-century Newtonians.
(Moreover, it is a nice question, to which we shall return at length below, how far the
principle of the conservation of kinetic plus potential energy, with its attendant [ Pobierz całość w formacie PDF ]
zanotowane.pl doc.pisz.pl pdf.pisz.pl szkicerysunki.xlx.pl
particular, the crucial second premiss of the causal argument, the completeness of
physics, would seem already to have been available to Leibniz. So doesn't this mean that
everything needed to appreciate the causal argument was already to hand in the second
half of the seventeenth century, long before the rise of twentieth-century materialism?
Well, it was but only on the assumption that Leibniz gives us the correct dynamics.
However, Leibniz's physical theories were quickly eclipsed by those of Newton, and this
then reopened the whole issue of the completeness of physics.
The central point here is that Newton allowed forces other than impact. Leibniz, along
with Descartes and all other pre-Newtonian proponents of the mechanical philosophy ,
took it as given that all physical action is by contact. They assumed that the only possible
cause of a change in a physical body's motion is the impact of another physical body. (Or
more precisely, as we are telling the story, Descartes supposed that the only possible non-
mental cause of physical change is impact, and Leibniz then argued that mental causes
other than impact are not possible either, if the conservation of momentum and energy
are to be respected.)
Newtonian mechanics changed the whole picture. This is because Newton did not take
impact as his basic model of dynamic action. Rather, his basic notion was that of an
impressed force. Rather than thinking of force as something inside a body which might
be transferred to other bodies in impact, as did all his contemporaries (and indeed most of
his successors for at least a century4 ), Newton thought of forces as disembodied entities,
acting on the affected body from outside. An impressed force consists in the action only,
and remains no longer in the body when the action is over . Moreover, impressed forces
are of different origins, as from percussion, from pressure, from centripetal force
(Newton 1966 [1686]: 2, definition IV). Gravity was the paradigm. True, the force of
gravity always arose from the presence of massive bodies, but it pervaded space, acting
on anything that might be there, so to speak, with a strength as specified by the inverse
square law.
Once disembodied gravity was allowed as a force distinct from the action of impact, then
there was no principled barrier to other similarly disembodied special forces, such as
chemical forces or magnetic forces or forces of cohesion (cf. Newton 1952 [1704]:
queries 29 31) or indeed vital and mental forces.
Nothing in classical Newtonian thinking rules out special mental forces. While Newton
has a general law about the effects of his forces (they cause proportional changes in the
velocities of the bodies they act on), there is no corresponding general principle about the
causes of such forces. True, gravity in particular is governed by the inverse square law,
which fixes gravitational forces as a function of the location of bodies with mass. But
there is no overarching principle dictating how forces in general arise. This opens up the
possibility that there may be sui generis mental forces, which would mean that
Newtonian physics, unlike Leibnizian physics, is not physically complete. Some physical
processes could have non-physical mental forces among their causal antecedents. (Some
readers may be feeling uneasy about the way in which the completeness of physics has
now turned into an issue about what forces exist. I shall address this issue at the end of
this section.)
The switch from a pure impact-based mechanical philosophy to the more liberal world of
Newtonian forces undermined Leibniz's argument for the completeness of physics.
Leibniz could hold that the principles governing the physical world leave no room for
mental acts to make a difference because he had a simple mechanical picture of the
physical world. Bodies preserve their motion in any given direction until they collide, and
then they obey the laws of impact. The Newtonian world of disembodied forces is far less
pristine, and gives no immediate reason to view physics as complete.
You might think that the conservation laws of Newtonian physics would themselves
place constraints on the generation of forces, in such a way as to restore the completeness
of physics. But this would be a somewhat anachronistic thought. Conservation laws did
not play a central role in Newtonian thinking at least not in that of Newton himself and
his immediate followers. True, Newton's mechanics does imply the conservation of
momentum. This falls straight out of his Third Law, which requires that action and
reaction are always equal. But it is a striking feature of Newtonian dynamics that there is
no corresponding law for energy.5
Of course, as we shall see in the next section, the principle of the conservation of kinetic
and potential energy in all physical processes did eventually become part of the
Newtonian tradition, and this does impose a general restriction on possible forces, a
restriction expressed by the requirement that all forces should be conservative . But this
came much later, in the middle of the nineteenth century, and so had no influence on the
range of possible forces admitted by seventeenth- or eighteenth-century Newtonians.
(Moreover, it is a nice question, to which we shall return at length below, how far the
principle of the conservation of kinetic plus potential energy, with its attendant [ Pobierz całość w formacie PDF ]