Once matter is imagined as an immense collection of tiny bodies, another question naturally arises. What are those bodies themselves like?
One answer proved remarkably influential.
They are imagined as hard.
Compact.
Distinct.
They possess definite boundaries.
They move.
They collide.
They rebound.
In short, they are conceived rather like exceedingly small billiard balls.
The image is immediately intuitive.
A billiard ball is an object with a clear identity.
It occupies a definite position.
It excludes other objects from occupying the same place.
Its behaviour appears stable and predictable.
It changes its motion through interaction with other bodies, yet it remains recognisably the same object throughout.
These familiar intuitions become available for thinking about matter at the smallest scales.
The billiard-ball metaphor refines the earlier corpuscular imagination.
The tiny bodies are no longer simply numerous.
They acquire a more definite character.
Each becomes an individual object with well-defined boundaries and persistent identity.
Matter is no longer merely composed of many things.
It is composed of many self-contained things.
This introduces a particularly powerful style of explanation.
Complex physical phenomena can be understood as the cumulative result of innumerable local interactions.
Large-scale behaviour becomes the organised consequence of countless small encounters.
The movement of the whole is explained through the movements of its constituent parts.
The metaphor also gives collision a central conceptual role.
Change occurs because one body encounters another.
Interaction is imagined through contact.
Cause becomes something transmitted from one object to the next through physical encounter.
The world acquires a distinctly mechanical character.
Another consequence of this image is the strengthening of individual persistence.
Each constituent body remains itself while participating in different interactions.
Its history may change.
Its position may change.
Its motion may change.
Yet the object itself is imagined as enduring through these alterations.
Identity becomes attached to the individual body rather than to the material as a whole.
The metaphor also encourages a particular understanding of complexity.
Complex systems need not possess fundamentally complex constituents.
A vast number of relatively simple objects, interacting according to stable principles, may generate remarkably intricate behaviour.
The richness of the world is imagined as emerging from the organised activity of simple parts.
Yet, as with every metaphor in this series, the billiard-ball image carries assumptions that often become invisible through familiarity.
One of these is the assumption of sharp individuation.
Each body is conceived as clearly distinguishable from every other.
Its boundaries are definite.
Its identity is unambiguous.
The world becomes naturally divisible into separate things.
Another assumption concerns local interaction.
Objects influence one another through encounters between individuals.
The conceptual emphasis falls upon discrete interactions rather than upon the collective organisation of the whole.
The individual remains the primary unit of explanation.
A further implication is that matter comes to be imagined as fundamentally solid.
Even when these constituent bodies are far too small to observe directly, they inherit the intuitive qualities of ordinary solid objects.
Hardness, persistence, and exclusion quietly become part of the conceptual landscape.
The familiar world is projected into the invisible one.
Taken together, these features make the billiard-ball metaphor extraordinarily successful.
It provides a clear and highly intelligible picture of physical interaction.
It allows complicated systems to be understood through the behaviour of simple constituents.
And it offers an image whose familiarity makes it remarkably easy to think with.
Yet that same familiarity can make the metaphor difficult to notice.
The image of tiny solid objects becomes so natural that it begins to appear self-evident.
We no longer recognise it as a particular way of imagining matter.
It comes to seem as though the invisible world simply consists of miniature versions of the objects we already know.
The metaphor becomes transparent through use.
At that point, something subtle occurs.
The conceptual qualities of everyday objects begin to migrate into the foundations of physical explanation.
Persistence.
Boundaries.
Collision.
Solidity.
These cease to appear as features of the metaphor.
They begin to appear as features of matter itself.
The question, then, is not whether this image was useful.
Its influence upon the development of physics is unmistakable.
The more interesting question is what kinds of explanation become possible once matter is imagined as an immense population of tiny, self-contained objects.
And what possibilities become more difficult to conceive while this image quietly organises physical thought?
We will not attempt to answer those questions here.
Instead, we simply note that the imagination of matter did not remain content with perfectly simple bodies.
The tiny objects themselves would gradually acquire an internal richness.
They would cease to be imagined as merely hard constituents.
They would become identifiable kinds of constituent.
Not simply tiny bodies.
But atoms.
And with that shift, the imagination of matter would begin another remarkable transformation.