The first stage is the discovery of perceptual objects. These objects are first known by the instinctive 'conveyance' of abnormal perceptions of sense-objects associated with normally perceived sense-objects. The test of alternative possibilities of normal perception and the discovery of a permanent character in the association which can be expressed independently of any particular percipient event decides between delusive perceptual objects and physical objects. 61.5 The introduction of physical objects enables us in considering the characters of events to sweep aside the boundless eccentricities of abnormal perceptions. We are still at the stage of apparent characters, but rules have been attained, either by instinctive practice or by the exercise of intelligence or by the interplay between the two, by which we know what to attend to and what to discard in judging the character of an event from the situations of sense-objects. A physical object is the apparent character of its situation. Physical objects are found to be 'material' objects. 61.6 Science now intervenes with the express purpose of exhibiting our perceptions as our awareness of the characters of events and of relations between characters of events. All perceptions are included in the scope of this aim of science, namely, including abnormally perceived sense-objects and delusive perceptual objects. 61.7 The origin of the concept of causation (in this application of the term) is now manifest. It is that of the part explaining the whole—or, avoiding this untechnical use of 'part' and 'whole,' it is that of some explaining all. For the physical objects were obtained by discarding abnormalities, and physical objects express the characters of events, and all our perceptions (including abnormalities) arise from awareness of these characters. 61.8 But physical objects fail to satisfy the requirements of science. They lack definiteness and permanence, and are not adequate for the purposes of explanation. Now the characters of their mutual relations disclose further permanences recognisable in events and among these are the scientific objects. The gradual recognition of these permanences was at first the slow product of civilised thought without conscious direction. As regards their conscious discovery various stages may be discerned in scientific history, which sum up the previous growth of ideas and initiate new epochs. One stage is marked by Archimedes' discovery of specific gravity, and another by Newton's discovery of mass. The simplicity of what, in its relation to appearance, is so abstract was then beginning to be discovered, and also its permanence and self-sufficiency as a quality of events. A third stage is the introduction of the concept of molecules and atoms by Dalton's atomic theory. Finally there arose the concepts respecting the ether, which we here construe as meaning the concept of events in space empty of appearances.