The first principle of cardiology is that the function of the heart is to pump blood through the circulatory system.

• Epistemologically, we can understand the structure of the heart only in terms of that function. If someone's heart fails to perform the blood-pumping function after a time, we regard that heart as abnormal and seek to discover how and why it differs from the normally functioning heart. Thus the proper function of the heart is our starting or reference point, even for the study of unhealthy hearts.

• Metaphysically, the blood-pumping function of the heart is the primary determinant of the organ's nature. Conceivably, early in the evolutionary process the heart might have originated from some proto-organ serving some other function, which perhaps itself arose from some accidental mutation. Nevertheless, in its present form, the heart is so admirably adapted to its blood-pumping function, that we can say that that function explains the nature and existence of the organ. (Note that because existence and identity are indivisible (pp. 1.4:3 ff.), whatever explains the nature of a thing can also be said to explain its existence.)