Sentence:  In the consideration of the faculties and impulses—of the "prima mobilia" of the human soul, the phrenologists have failed to make room for a propensity which, although obviously existing as a radical, primitive, irreducible sentiment, has been equally overlooked by all the moralists who preceded them. [...] We could not understand, that is to say, we could not have understood, had the notion of this <WSD>"primum mobile</WSD>" ever obtruded itself;— [...] 
Meaning of the word marked with <WSD> tags:  The person or thing that is the main impetus for some action; a driving force.