If you sit down, elbows on a table, head between hands, covering your ears so that the noises of the room are cut off, you can hear a weak rumble. Especially if you give your hands the shape of a shell, taking the lobe and the pinna of your ear between your hand and thumb, and closing them gently against your head at the front side, it is as if you are hearing a waterfall at some distance. This is no illusory interpretation of some background noise. You are in fact hearing the fast current of a flowing fluid – the stream of your own blood. The circulation of the blood is a directly observable fact, apparently accessible to anyone who cares to listen. It even seems straightforward and is less ambiguous than the movements of the Earth and the Sun or the planets. There are no alternative interpretations. You cannot choose, as in the case of sunset, where you can perceive the Sun as moving slowly behind the horizon or the Earth as turning slowly away from the Sun. It is almost a case of direct perception, allowing no choice in interpretation.

The discovery of the circulation of blood was reported by William Harvey in 1628 in his De motu cordis, not as a simple matter of observation but as the outcome of a series of ingenious inferences and experiments.

Considering the familiarity of the domain and the relative unambiguity of the observation, it is astonishing that this discovery could take so long.