IN the chapter entitled Social Explanation we distinguished two interpretations of the query ‘What is its function?’ One, we said, takes the form ‘What is it intended to do – what purpose does it serve? This interpretation presents no problems different from those already discussed in the chapter on intentions. It is the other interpretation which now concerns us, the one which takes the sense of the question to be ‘What part does it play in the operation of the system?’ A typical example is: ‘The function of the heart is to circulate the blood.’