Sentence:  In some individuals optimism may become quasi-pathological. The capacity for even a transient sadness or a momentary humility seems cut off from them as by a kind of congenital <WSD>anæsthesia</WSD>. 
Meaning of the word marked with <WSD> tags:  The loss or prevention of sensation, as caused by anesthesia, lesion in the nervous system or other physical abnormality.