This distinction between what is 'perceived' and what is 'really perceived' does not solely arise from time differences. For example Alciphron, in Berkeley's dialogue, sees a crimson cloud. Suppose that he had seen the cloud in a mirror. He would have 'seen' crimson as situated in an event behind the mirror, but he would have 'really seen' the cloud behind him. These examples show that the property of being the situation of a sense-object for a given percipient event is in some respects a trivial property of an event. Yet, in other respects, it is very important; namely, it is important for the consciousness associated with the percipient event. The situations of sense-objects form the whole basis of our knowledge of nature, and the whole structure of natural knowledge is founded on the analysis of their relations. 23.6 The definiteness for human percipients of the situations of sense-objects varies greatly for different types of such objects. The sound of a bell is in the bell, it fills the room, and jars the brain. The feeling a push against a hard rock is associated with the rock as hardness and with the body as effort, where hardness and effort are objects of sense. This duplication of sense-objects is a normal fact in perception, though one of the two associated pair, either the one in the body or the one in a situation separated from the body, is usually faintly perceived and indeterminate as to situation. 23.7 The relationship between a sense-object and nature, so far as it is restricted to one percipient event and one situation, is completed by the conditioning events. The special characters in which they enter into that relationship depend on the particular case under consideration. Conditioning events may be divided into two main classes which are not strictly discriminated from each other. Namely, there are the events which are 'passive' conditions and the events which are 'active' conditions. An event which is an active condition is a cause of the occurrence of the sense-object in its situation for the percipient event; at least, it can be so termed in one of the many meanings of the word 'cause.' Also space and time are presupposed as the setting within which the particular events occur. But space and time are merely expressive of the relations of extension among the whole ether of events. Thus this presupposition of space and time really calls in all events of all nature as passive conditions for that particular perception of the sense-object. The laws of nature express the characters of the active conditioning events and of the percipient events, which issue in the recognition of a definite sense-object in an assigned situation.