(we do not say always) that there are a number of other
particulars differing from this one in gradually increasing degrees.
Those (or some of those) that differ from it only very slightly will
be found to differ approximately according to certain laws which may be
called, in a generalized sense, the laws of "perspective"; they include
the ordinary laws of perspective as a special case. This approximation
grows more and more nearly exact as the difference grows less; in
technical language, the laws of perspective account for the differences
to the first order of small quantities, and other laws are only
required to account for second-order differences. That is to say, as the
difference diminishes, the part of the difference which is not according
to the laws of perspective diminishes much more rapidly, and bears to
the total difference a ratio which tends towards zero as both are made
smaller and smaller. By this means we can theoretically collect together
a number of particulars which may be defined as the "aspects" or
"appearances" of one thing at one time. If the laws of perspective were
sufficiently known, the connection between different aspects would be
expressed in differential equations.

This gives us, so far, only those particulars which constitute one thing
at one time. This set of particulars may be called a "momentary
thing." To define that series of "momentary things" that constitute
the successive states of one thing is a problem involving the laws of
dynamics. These give the laws governing the changes of aspects from
one time to a slightly later time, with the same sort of differential
approximation to exactness as we obtained for spatially neighbouring
aspects through the laws of perspective. Thus a momentary thing is a set
of particulars, while a thing (which may be identified with the whole
history of the thing) is a series of such sets of particulars.
The particulars in one set are collected together by the laws of
perspective; the successive sets are collected together by the laws
of dynamics. This is the view of the world which is appropriate to
traditional physics.

The definition of a "momentary thing" involves problems concerning time,
since the particulars constituting a momentary thing will not be all
simultaneous, but will travel outward from the thing with the velocity
of light (in case the thing is in vacuo). There are complications
connected with relativity, but for our present purpose th