We now know that the external world exists. But what is its nature? Is it as we perceive it to be? Or is it different? This is what we'll address in this lecture. We perceive the external world through our senses, so let's see if our sensations correspond to qualities that naturally inhere in matter.

The qualities of matter that are known to us through our senses can be divided into two distinct classes.

Not all objects have qualities of the first class, for we can conceive of objects independent of them. These are only forms of other properties of matter, and thus we call them secondary qualities – for example, heat, color, taste, odor, etc. Clearly there are objects that have neither taste nor odor, and we can imagine an object lacking color or heat. Science tells us that sound and color are only varieties of movement, and the same might be said for the other secondary qualities.

Qualities of the other class – primary qualities – have the opposite characteristics. All objects have such qualities. We can't conceive of an object independent of them. All secondary qualities can be reduced to these primary qualities, whereas they themselves are irreducible.

There are only two primary qualities – extension and movement. All bodies are extended and mobile, as we can't conceive of an object which would be unextended or immovable.