ts unaccompanied by
any perception of physical objects. This lack of reciprocity in the
relations between sense-objects and physical objects is fatal to the
scholastic natural philosophy.

There is a great difference in the _roles_ of the situations of
sense-objects and physical objects. The situations of a physical object
are conditioned by uniqueness and continuity. The uniqueness is an ideal
limit to which we approximate as we proceed in thought along an
abstractive set of durations, considering smaller and smaller durations
in the approach to the ideal limit of the moment of time. In other
words, when the duration is small enough, the situation of the physical
object within that duration is practically unique.

The identification of the same physical object as being situated in
distinct events in distinct durations is effected by the condition of
continuity. This condition of continuity is the condition that a
continuity of passage of events, each event being a situation of the
object in its corresponding duration, can be found from the earlier to
the later of the two given events. So far as the two events are
practically adjacent in one specious present, this continuity of passage
may be directly perceived. Otherwise it is a matter of judgment and
inference.

The situations of a sense-object are not conditioned by any such
conditions either of uniqueness or of continuity. In any durations
however small a sense-object may have any number of situations separated
from each other. Thus two situations of a sense-object, either in the
same duration or in different durations, are not necessarily connected
by any continuous passage of events which are also situations of that
sense-object.

The characters of the conditioning events involved in the ingression of
a sense-object into nature can be largely expressed in terms of the
physical objects which are situated in those events. In one respect this
is also a tautology. For the physical object is nothing else than the
habitual concurrence of a certain set of sense-objects in one situation.
Accordingly when we know all about the physical object, we thereby know
its component sense-objects. But a physical object is a condition for
the occurrence of sense-objects other than those which are its
components. For example, the atmosphere causes the events which are its
situations to be active conditioning events in the transmission of
sound. A mirror which is itself a physical