Sentence:  Why is it, then, that there is on the whole a preponderance among mankind of rational opinions and rational conduct? ... it is owing to a quality of the human mind, the source of everything respectable in man either as an intellectual or as a moral being, namely, that his errors are <WSD>corrigible</WSD>. 
Meaning of the word marked with <WSD> tags:  Able to be corrected or set right.