Routledge History of Philosophy Volume IV

RENAISSANCE AND SEVENTEENTH-CENTURY RATIONALISM 117
à due nuove scienze of 1638, another dialogue between the three friends Salviati,
Sagredo and Simplicio, but this time steering well clear of the dangerous
question of the motion of the Earth.
Galileo’s strategy in considering local motions (that is, motions according to
place rather than to quality or quantity) was to split them into two components,
horizontal and vertical. In an amusing exchange in the Dialogo, the unwary
Simplicio is trapped by Salviati into admitting that a perfectly shaped ball rolling
on a perfectly smooth horizontal surface would continue its motion indefinitely
and with uniform speed. The context is the question of the behaviour of stones
let drop from the mast of a moving ship (relevant of course to arguments about
Copernicus). The Aristotelian Simplicio had demanded experiment, but Salviati
was adamant that without experiment he could demonstrate that they would fall
at the bottom of the mast and not be left behind by the motion of the ship. For
this purpose he made use of an ingenious thought experiment with inclined
planes. Passages like this led Alexandre Koyré and others to lay great stress on
what they saw as Galileo’s Platonic streak, and heated controversy continues
concerning the importance of experiment in Galileo’s work as regards both the
context of discovery and the context of justification, but no serious scholar would
now go to Koyré’s extremes in denigrating its role.
Although Salviati demonstrated to his companions’ satisfaction the uniform
speed of unimpeded horizontal motion, a complication remained, for such a
motion is in fact in a circle about the centre of the Earth. In the Dialogo Galileo
made some play with this to illustrate the circle’s superiority over the straight
line, but in the Discorsi he quickly approximates this with a straight line, and in
doing so appeals to Archimedes (a particular hero of his), who in his On Floating
Bodies had said that verticals could be treated as parallel even though in fact they
all pointed towards the Earth’s centre.
Vertical motion needed two considerations. In Day 1 of the Discorsi Salviati
argued that in a vacuum all bodies, whatever their density, would fall with the
same speed. A vacuum was unrealizable in practice, and so his main strategy
employed thought experiments concerning ever rarer media, but he does cite
somewhat exaggeratedly some actual experimentation with pendulums: we
certainly do not need to consider here objects dropped from the Leaning Tower of
Pisa. His more famous, and much discussed, argument about falling bodies
occurs in Day 3, and concerns the acceleration of falling bodies. That they do
accelerate had been known from time immemorial, but since at least the time of
Aristotle most discussion had centred on why they did so. Salviati would have
none of this.
The present does not seem to me to be an opportune time to enter into an
investigation of the cause of the acceleration of natural motion, concerning
which various philosophers have produced various opinions, some
reducing it to approach to the centre, others to there remaining successively
less parts of the medium to be divided, others to a certain extrusion of the