Qualitative novelty in seventeenth-century science: Hydrostatics from Stevin to Pascal

• The treatise on hydrostatics by Stevin was formulated in the style of Euclid.
• Stevin illegitimately employed assumptions not included in his postulates.
• Pascal's treatise on hydrostatics introduced a novel concept of pressure.
• The new concept emerged as the result of interplay between theory and experiment.
• The case for Pascal's hydrostatics was experimental rather than Euclidean.

Two works on hydrostatics, by Simon Stevin in 1586 and by Blaise Pascal in 1654, are analysed and compared. The contrast between the two serves to highlight aspects of the qualitative novelty involved in changes within science in the first half of the seventeenth century. Stevin attempted to derive his theory from unproblematic postulates drawn from common sense but failed to achieve his goal insofar as he needed to incorporate assumptions involved in his engineering practice but not sanctioned by his postulates. Pascal's theory went beyond common sense by introducing a novel concept, pressure. Theoretical reflection on novel experiments was involved in the construction of the new concept and experiment also provided important evidence for the theory that deployed it. The new experimental reasoning was qualitatively different from the Euclidean style of reasoning adopted by Stevin. The fact that a conceptualization of a technical sense of pressure adequate for hydrostatics was far from obvious is evident from the work of those, such as Galileo and Descartes, who did not make significant moves in that direction.