The scientific study of man is the most difficult of all branches of knowledge.
 Oliver Wendell Holmes Sr., The Poet at the Breakfast-Table (1872). copy citation

Context

“Do you mean that you would not give the cup of cold water for the sake simply and solely of the poor, suffering fellow-mortal, as willingly as you now do, professing to give it for the sake of Him who is not thirsty or in need of any help of yours? We must ask questions like this, if we are to claim for our common nature what belongs to it. The scientific study of man is the most difficult of all branches of knowledge. It requires, in the first place, an entire new terminology to get rid of that enormous load of prejudices with which every term applied to the malformations, the functional disturbances, and the organic diseases of the moral nature is at present burdened.” source