Expansion is like a three‑dimensional solid whose lengths can be 'turned every way' to make figure, … — John Locke

Expansion is like a three‑dimensional solid whose lengths can be 'turned every way' to make figure, breadth, and thickness, whereas duration is like the length of a single straight line extended to infinity, incapable of multiplicity, variation, or figure, and serving as one common measure in which all existing things equally partake of the same present moment; we cannot clearly conceive real existence with a total negation of either expansion or duration, though we are ignorant of how spirits relate to space and only know that bodies each occupy and exclude others from their proper portion of it.

By John Locke, from An Essay Concerning Human Understanding

Key Arguments

He asserts a 'manifest difference' between duration and expansion: the ideas of length of expansion 'are turned every way, and so make figure, and breadth, and thickness', but duration is only 'the length of one straight line, extended in infinitum, not capable of multiplicity, variation, or figure'.
Duration is 'one common measure of all existence whatsoever, wherein all things, whilst they exist, equally partake', since 'this present moment is common to all things that are now in being' so that 'we may truly say, they all exist in the same moment of time'.
Locke confesses his ignorance about whether angels and spirits have any analogue to spatial expansion, noting the limits of human understanding which is 'suited to our own preservation' rather than to the full 'reality and extent of all other beings'.
He suggests that for us it is almost as hard to conceive real existence with a perfect negation of all expansion as it is with a perfect negation of all duration.
He concludes that we do not know what spirits have to do with space or how they are present in it; what we do know is restricted to bodies, each of which 'singly possess its proper portion' of space according to its solid extent, thereby excluding all other bodies from that portion as long as it remains there.

Source Quotes

Duration is as a line, expansion as a solid. But there is this manifest difference between them — That the ideas of length which we have of expansion are turned every way, and so make figure, and breadth, and thickness; but duration is but as it were the length of one straight line, extended in infinitum, not capable of multiplicity, variation, or figure; but is one common measure of all existence whatsoever, wherein all things, whilst they exist, equally partake. For this present moment is common to all things that are now in being, and equally comprehends that part of their existence, as much as if they were all but one single being; and we may truly say, they all exist in the same moment of time.

But there is this manifest difference between them — That the ideas of length which we have of expansion are turned every way, and so make figure, and breadth, and thickness; but duration is but as it were the length of one straight line, extended in infinitum, not capable of multiplicity, variation, or figure; but is one common measure of all existence whatsoever, wherein all things, whilst they exist, equally partake. For this present moment is common to all things that are now in being, and equally comprehends that part of their existence, as much as if they were all but one single being; and we may truly say, they all exist in the same moment of time. Whether angels and spirits have any analogy to this, in respect to expansion, is beyond my comprehension: and perhaps for us, who have understandings and comprehensions suited to our own preservation, and the ends of our own being, but not to the reality and extent of all other beings, it is near as hard to conceive any existence, or to have an idea of any real being, with a perfect negation of all manner of expansion, as it is to have the idea of any real existence with a perfect negation of all manner of duration.

For this present moment is common to all things that are now in being, and equally comprehends that part of their existence, as much as if they were all but one single being; and we may truly say, they all exist in the same moment of time. Whether angels and spirits have any analogy to this, in respect to expansion, is beyond my comprehension: and perhaps for us, who have understandings and comprehensions suited to our own preservation, and the ends of our own being, but not to the reality and extent of all other beings, it is near as hard to conceive any existence, or to have an idea of any real being, with a perfect negation of all manner of expansion, as it is to have the idea of any real existence with a perfect negation of all manner of duration. And therefore, what spirits have to do with space, or how they communicate in it, we know not.

And therefore, what spirits have to do with space, or how they communicate in it, we know not. All that we know is, that bodies do each singly possess its proper portion of it, according to the extent of solid parts; and thereby exclude all other bodies from having any share in that particular portion of space, whilst it remains there. 12.

Key Concepts

But there is this manifest difference between them — That the ideas of length which we have of expansion are turned every way, and so make figure, and breadth, and thickness;
but duration is but as it were the length of one straight line, extended in infinitum, not capable of multiplicity, variation, or figure; but is one common measure of all existence whatsoever, wherein all things, whilst they exist, equally partake.
For this present moment is common to all things that are now in being, and equally comprehends that part of their existence, as much as if they were all but one single being; and we may truly say, they all exist in the same moment of time.
perhaps for us, who have understandings and comprehensions suited to our own preservation, and the ends of our own being, but not to the reality and extent of all other beings, it is near as hard to conceive any existence, or to have an idea of any real being, with a perfect negation of all manner of expansion, as it is to have the idea of any real existence with a perfect negation of all manner of duration.
All that we know is, that bodies do each singly possess its proper portion of it, according to the extent of solid parts; and thereby exclude all other bodies from having any share in that particular portion of space, whilst it remains there.

Context

Book II, chapter XV, section 11, where Locke contrasts the geometrical character of expansion with the linear character of duration, develops the notion of the common present moment, and reflects on our inability to conceive existence without either expansion or duration, especially with respect to spirits.