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.
CHAPTER V. CONCERNING THE MOVING FORCE OF MATTER.
The ancients, persuaded that there is no body without a moving force, regarded the substance of bodies as composed of two primitive attributes. It was held that, through one of these attributes, this substance has the capacity for moving and, through the other, the capacity for being moved.[88] As a matter of fact, it is impossible not to conceive these two attributes in every moving body, namely, the thing which moves, and the same thing which is moved.