4 PROGRESS OF HUMAN KNOWLEDGE. 



stated. These conditions, however, must not be re- 

 ceived without limitation. The lines between the 

 known and unknown between the possible and im- 

 possible are not always drawn as distinctly as these 

 terms assume. The boundaries are perpetually shifting, 

 and at no period so rapidly as in our own day. Arago 

 says : ' Celui qui, en dehors des mathematiques pures, 

 prononce le mot impossible ', commet une imprudence.' 

 Twenty years ago who would not have declared it 

 impossible to detect and define metals in the sun and 

 fixed stars, or to transmit within a few minutes a 

 message from London to New York ? Physical science 

 in its progress affords many such examples, though 

 none more striking. But even here, and much more 

 still in the physiology of life, in mental philosophy and 

 theology, there are problems insuperable in their very 

 nature, which men of highest capacity, willingly or 

 unwillingly, feel as such, and submit to the limitation. 

 The field, indeed, is ample enough for all labourers in 

 physical science within the boundary thus defined ; 

 and is ever enlarging in its compass. Each discovery 

 made shadows forth new questions to be solved, often 

 of higher import than the discovery which suggested 

 them. Those most deeply versed in the phenomena of 

 the natural world best apprehend the large future which 

 lies before them. It is only in the dark circle of 

 ignorance that knowledge is regarded as certain and 

 complete. 



The ancient and mediaeval philosophers, careless, or 

 unable to define the true objects and limits of enquiry, 

 have left us a large legacy of vague or incongruous 



