Although we have no idea of the essence of matter, we can not refuse to
admit the existence of the properties which our senses discover in it.
I open my eyes, and I see around me only matter, or the
extended. Extension is then a property which always belongs to all
matter, which can belong to matter alone, and which therefore is
inseparable from the substance of matter.
This property presupposes three dimensions in the substance of bodies,
length, width, and depth. Truly, if we consult our knowledge, which is
gained entirely from the senses, we cannot conceive of matter, or the
substance of bodies, without having the idea of a being which is at
the same time long, broad, and deep; because the idea of these three
dimensions is necessarily bound up with our idea of every magnitude
or quantity.
Those philosophers who have meditated most concerning matter do not
understand by the extension of this substance, a solid extension
composed of distinct parts, capable of resistance. Nothing is united,
nothing is divided in this extension; for there must be a force which
separates to divide, and another force to unite the divided parts. But
in the opinion of these physical philosophers matter has no actually
active force, because every force can come only from movement, or from
some impulse or tendency toward movement, and they recognize in matter,
stripped of all form by abstraction, only a potential moving force.
This theory is hard to conceive, but given its principles, it is
rigorously true in its consequences. It is one of those algebraic
truths which is more readily believed than conceived by the mind.
The extension of matter is then but a metaphysical extension, which
according to the idea of these very philosophers, presents nothing to
affect our senses. They rightly think that only solid extension can
make an impression on our senses. It thus seems to us that extension
is an attribute which constitutes part of the metaphysical form,
but we are far from thinking that extension constitutes its essence.
However, before Descartes, some of the ancients made the essence of
matter consist in solid extension. But this opinion, of which all the
Cartesians have made much, has at all times been victoriously combated
by clear reasons, which we will set forth later, for order demands that
we first examine to what the properties of extension can be reduced.