The close connection between solidity and matter makes it not only possible, but necessary, to distinguish between matter and extension as against the Cartesians, who had identified them. In particular Locke notes three differences between these notions. Extension includes neither solidity nor resistance; its parts are inseparable from one another both really and mentally, and are immovable; while matter has solidity, its parts are mutually separable, and may be moved _in_ space. From this distinction between space and matter it follows, according to Locke, that there is such a thing as a vacuum, or that space is not necessarily a plenum of matter. Matter is that which fills space; but it is entirely indifferent to space whether or not it is filled. Space is occupied by matter, but there is no essential relation between them. Solidity is the essence of matter; emptiness is the characteristic of space. "The idea of space is as distinct from that of solidity as it is from that of scarlet color. It is true, solidity cannot exist without extension, neither can scarlet color exist without extension; but this hinders not that they are _distinct ideas_."
Thus there is fixed for us the idea of space as well as of matter. It is a distinct idea; that is, absolute or independent in itself, having no intrinsic connection with phenomena _in_ space. Yet it is got through the senses. How that can be a matter of sensation which is not only not material, but has no connection in itself with matter, Locke does not explain. He thinks it sufficient to say that we see distance between bodies of different color just as plainly as we see the colors. Space is, therefore, a purely immediate idea, containing no more organic relation to intelligence than it has to objects. We get the notion of time as we do that of space, excepting that it is the observation of internal states and not of external objects which furnishes the material of the idea. Time has two elements,--succession and duration. "Observing what passes in the mind, how of our ideas there in train some constantly vanish, and others begin to appear, we come by the idea of succession, and by observing a distance in the parts of this succession we get the idea of duration." Whether, however, time is something essentially empty, having no relation to the events which fill it, as space is essentially empty, without necessary connection with the objects which fill it, is a question Locke does not consider. In fact, the gist of his ideas upon this point is as follows: there is actually an objective space or pure emptiness; employing our senses, we get the idea of this space. There is actually an objective time; employing reflection, we perceive it. There is not the slightest attempt to form a philosophy of them, or to show their function in the construction of an intelligible world, except in the one point of the absolute independence of matter and space.