Sentence:  For that is the characteristic of a <WSD>mercenary</WSD>, who acts through interest, rather than of an affectionate child who acts through love. There is a great difference, say they, between the service of a slave, the service of a <WSD>mercenary</WSD> or hireling, and the service of a child. 
Meaning of the word marked with <WSD> tags:  One motivated by gain, especially monetary.