Galilei, Galileo - The systems of the world

Galilei, Galileo, The systems of the world, 1661

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is its motion. Having confirmed theſe motions, he proceeds ſaying,
that of natural bodies ſome being ſimple, and ſome compoſed of
them (and he calleth ſimple bodies thoſe, that have a principle
of motion from nature, as the Fire and Earth) it follows that
ſimple motions belong to ſimple bodies, and mixt to the com
pound; yet in ſuch ſort, that the compounded incline to the part
predominant in the compoſition.

Local motion of
three kinds, right,
circular, & mixt.

Circular, and
ſtreight motions
are ſimple, as pro
ceeding by ſimple
lines.

Ad medium, à me
dio, & circa medi
um.

SAGR. Pray you hold a little Salviatus, for I find ſo many
doubts to ſpring up on all ſides in this diſcourſe, that I ſhall be
conſtrained, either to communicate them if I would attentively
hearken to what you ſhall add, or to take off my attention from
the things ſpoken, if I would remember objections.

SALV. I will very willingly ſtay, for that I alſo run the ſame
hazard, and am ready at every ſtep to loſe my ſelf whilſt I ſail be
tween Rocks, and boiſterous Waves, that make me, as they ſay, to
loſe my Compaſs; therefore before I make them more, propound
your difficulties.

The definition of
Nature, either im
perfect, or unſeaſo
nable, produced by
Ariſtotle.

SAGR. You and Ariſtotle together would at firſt take me a
little out of the ſenſible World, to tell me of the Architecture,
wherewith it ought to be fabricated; and very appoſitly begin to
tell me, that a natural body is by nature moveable, nature being
(as elſewhere it is defined) the principle of motion. But here I
am ſomewhat doubtfull why Ariſtotle ſaid not that of natural bo
dies, ſome are moveable by nature, and others immoveable, for
that in the definition, nature is ſaid to be the principle of Motion,
and Reſt; for if natural bodies have all a principle of motion,
either he might have omitted the mention of Reſt, in the definiti
on of nature: or not have introduced ſuch a definition in this place.
Next, as to the declaration of what Ariſtotle intends by ſimple
motions, and how by Spaces he determines them, calling thoſe ſim
ple, that are made by ſimple lines, which are onely the right, and

circular, I entertain it willingly; nor do I deſire to tenter the
inſtance of the Helix, about the Cylinder; which in that it is in e
very part like to it ſelf, might ſeemingly be numbred among ſim
ple lines. But herein I cannot concurre, that he ſhould ſo re
ſtrain ſimple motions (whilſt he ſeems to go about to repeat the
ſame definition in other words) as to call one of them the motion
about the medium, the others Surſum & Deorſum, namely up
wards and downward; which terms are not to be uſed, out of the
World fabricated, but imply it not onely made, but already in
habited by us; for if the right motion be ſimple, by the ſimplicity
of the right line, and if the ſimple motion be natural, it is made on
every ſide, to wit, upwards, downwards, backwards, forwards, to
the right, to the left, and if any other way can be imagined, pro
vided it be ſtraight, it ſhall agree to any ſimple natural body; or

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