Some men of science do indeed get over the difficulty by dealing only with the easy part of it
 G. K. Chesterton, What's Wrong with the World (1910). copy citation

Context

“But while this is granted, something further may be claimed. It is not merely true that these universal things are strange; it is moreover true that they are subtle. In the last analysis most common things will be found to be highly complicated. Some men of science do indeed get over the difficulty by dealing only with the easy part of it: thus, they will call first love the instinct of sex, and the awe of death the instinct of self-preservation. But this is only getting over the difficulty of describing peacock green by calling it blue.” source