24.6 The apprehension of an event as the situation of a physical object is our most complete perception of the character of an event. It represents a fundamental perception of a primary law of nature. It is solely by means of physical objects that our knowledge of events as active conditions is obtained, whether as generating conditions or as transmitting conditions. For example, the mirror is recognised as a physical object and its situation is the generating condition for that association of sense-objects; but its situation is also a transmitting condition for the sense-objects and delusive perceptual objects which are perceived as images behind it. Again, the prism is a physical object and its situation is a transmitting condition for the sense-object which is the spectrum. So far as it is directly perceived in its various situations, a physical object is a group of associations of sense-objects, each association being perceived or perceivable by a percipient object with an appropriate percipient event as its locus. But the object is more than the logical group; it is the recognisable permanent character of its various situations. 24.7 In spite of their insistence in perception these physical objects are infected with an incurable vagueness which had led speculative physics practically to cut them out of its scheme of fundamental entities. In the first place this vagueness arises from the unique situation of such an object within any small duration. The result is that the object is confounded with the event which is its situation. But a situation is prolonged in time, and a temporal part of that event is not the event itself. Now the object during ten seconds is not part of the object during one of these seconds. The object is always wholly itself during ten seconds or during one second. It is this train of thought which led to the introduction of the durationless instant of time as a fundamental fact, thus fatally confusing the philosophy of science. The error arose from not discriminating the object from its situation. The train of events which is the situation of the object through a prolonged stretch of time is not the unique object; it is the set of events with which the object has its unique association. The difficulty of this point of view arises from the fact that a temporal succession of events, each very similar to the others, ceases to mark for us the time-flux in comparison with the rhythmic changes of our bodies. The result is that in perceiving an unchanging cliff the recognition of permanence, i.e. of the object, overwhelms all other perception, the flux of events becoming a vague background owing to the absence of their demarcation in our perceptual experience.