i66 THE GRAMMAR OF SCIENCE 



is either possible or probable as a perception. Indeed, 

 the process or association by which we have reached our 

 conception may in itself suffice to exhibit its perceptual 

 impossibility or improbability. The appeal to experience 

 can alone determine whether a conception is possible as a 

 perception. For example, experience shows me that there 

 is a sensible limit to the visible and tangible ; hence a 

 point, valid as a conception, can never have a real existence 

 as a perception. I reach this conception of a point by 

 carrying to a limit in my imagination a process which 

 cannot be so carried in perception. Exactly of the same 

 character are my conceptions of infinite distance or infinite 

 number ; they are the conceptual limits to processes, 

 which may be started in perception, but cannot be carried 

 to a limit except in the imagination. Somewhat different 

 from perceptual impossibility is perceptual improbability. 

 I can conceive Her Majesty Queen Victoria walking down 

 Regent Street, but, tested by my experience of the past 

 actions of royalty, this association of conceptions is 

 hardly a perceptual probability. These instances may be 

 sufficient to indicate that what is improbable or impossible 

 in perception may be valid in conception. But we must 

 ever be careful to bear in mind that the ideality of the 

 conception, its existence outside thought, can only be 

 demonstrated by an appeal to perceptual experience. 

 The geometrician even asserts the phenomenal impossibility 

 of his points, lines, and surfaces; the physicist by no 

 means postulates the existence of atoms and molecules as 

 possible perceptions. Science is content for the present 

 to look upon these concepts as existing only in the sphere 

 of thought, as purely the product of man's mind. It does 

 not, like metaphysics or theology, demand any existence 

 in or beyond sense-impression for its conceptions until 

 experience has shown that the conceptual limit or associa- 

 tion can become a perceptual reality.^ The validity of 



^ Leverrier and Adams conceived a planet having a definite orbit as a 

 method of accounting for the irregularities perceived in the motions of Uranus. 

 Their conception might have been valid as a manner of describing these 

 irregularities, if Neptune itself had never been perceived — in other words, if 

 their conception had not become a perceptual reality. 



