our knowledge to understand what we see upon 

 these trails. But there is no such psychology, and 

 the assumption itself is a groundless one. Nature 

 is of one piece, and consistent throughout. The 

 drop is like the ocean, though it bears no ships on 

 its bosom ; the tear on a child's cheek breaks the 

 light into glorious color, as does the rainbow on 

 the spray of Niagara ; and the law that holds the 

 mountains fast sleeps in the heart of every grain of 

 sand on the seashore. When we wish to measure 

 the interstellar spaces we seek no new celestial unit, 

 but apply confidently our own yardstick ; and the 

 chemistry that analyzes a baby's food serves equally 

 well for the satellites of Jupiter. This is but an 

 analogy, to be sure, but it serves to guide us in the 

 realm of conscious life, which also seems of one 

 piece and under one law. Inspired writers of every 

 age have sought to comprehend even gods and 

 angels by the same human intelligence that they 

 applied to the ants and the conies, and for the same 

 reason, — that they possessed but one measure of 

 life. Love and hate, fear and courage, joy and grief, 

 pain and pleasure, want and satisfaction, — these 



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Preface 



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