Spencer, H. (1897). The relations of likeness and unlikeness. In H. Spencer, The principles of psychology, Vol. 2-1 (pp. 281-285). New York, NY, US: D Appleton & Company.
http://dx.doi.org/10.1037/12897-024
At length continued analysis has brought us down to the relations underlying not only all preceding relations, but all processes of thought whatever. From the most complex and most abstract inferences down to the most rudimentary intuitions, all intelligence proceeds by the establishment of relations of likeness and unlikeness. Duly to appreciate this truth, we must glance at the successive conclusions arrived at in preceding chapters. (PsycINFO Database Record (c) 2016 APA, all rights reserved)