hundredth of a second, to a thousandth of a
second, and so on. As we pass along such a series we approximate to an
ideal simplicity of structural relations of the pairs of events
successively considered, which ideal we call the spatial relations of
the Needle to the barge at some instant. Even these relations are too
complicated for us, and we consider smaller and smaller bits of the
Needle and of the barge. Thus we finally reach the ideal of an event so
restricted in its extension as to be without extension in space or
extension in time. Such an event is a mere spatial point-flash of
instantaneous duration. I call such an ideal event an 'event-particle.'
You must not think of the world as ultimately built up of
event-particles. That is to put the cart before the horse. The world we
know is a continuous stream of occurrence which we can discriminate into
finite events forming by their overlappings and containings of each
other and separations a spatio-temporal structure. We can express the
properties of this structure in terms of the ideal limits to routes of
approximation, which I have termed event-particles. Accordingly
event-particles are abstractions in their relations to the more concrete
events. But then by this time you will have comprehended that you cannot
analyse concrete nature without abstracting. Also I repeat, the
abstractions of science are entities which are truly in nature, though
they have no meaning in isolation from nature.

The character of the spatio-temporal structure of events can be fully
expressed in terms of relations between these more abstract
event-particles. The advantage of dealing with event-particles is that
though they are abstract and complex in respect to the finite events
which we directly observe, they are simpler than finite events in
respect to their mutual relations. Accordingly they express for us the
demands of an ideal accuracy, and of an ideal simplicity in the
exposition of relations. These event-particles are the ultimate elements
of the four-dimensional space-time manifold which the theory of
relativity presupposes. You will have observed that each event-particle
is as much an instant of time as it is a point of space. I have called
it an instantaneous point-flash. Thus in the structure of this
space-time manifold space is not finally discriminated from time, and
the possibility remains open for diverse modes of discrimination
according to the diverse circumstances of ob