This article in an attempt to identify the precise way in which Robert Boyle provided a mechanical account of the features that distinguish liquids and air from solids and from each other. In his pneumatics, Boyle articulated his notion of the ‘spring’ of the air for that purpose. Pressure appeared there only in a common, rather than in a technical, sense. It was when he turned to hydrostatics that Boyle found the need to introduce a technical sense of pressure to (...) capture the fluidity of water which, unlike air, lacked a significant spring. Pressure, understood as representing the state of a liquid within the body of it rather than at its surface, enabled Boyle to trace the transmission of hydrostatic forces through liquids and thereby give a mechanical account of that transmission according to his understanding of the term. This was a major step towards the technical sense of pressure that was to be adopted in Newton’s hydrostatics and in fluid mechanics thereafter. (shrink)