At the beginning of the 17th century the epistemological criteria too were about to change. There was a shift from the medieval distinction between sensible qualities (valid knowledge) and occult qualities (non-valid) to the Galileian one between primary quantifiable (valid) and secondary non-quantifiable (non-valid) qualities.
The problem was that primary qualities were often not directly sensible, and sometimes, as in the case of gravitation, definitely occult. The conflict between the two epistemologies was sharp. The so-called experimental method was not based on observation: the sun actually rises and sets, heavier bodies fall faster, the pendulum does not rise back to the same height. Galileo to the Aristotelian paradigm based on observation substituted a Platonic one based on the existence of mathematical laws. These laws were not directly observable, but their existence could be inferred on the basis of complex arguments and could be corroborated on the basis of complex experiments carried forward with new instruments. This epistemology and methodology won the battle in the field of " classical " sciences, but not in the measurement and quantification of the new " Baconian " fields, as in physiology, where priority was given to the elimination of the " occult " qualities. In these fields Leibnitian research programmes had a lasting and productive influence. Moreover it is to be remembered that in this period took place the critic of induction by Hume and a deep analysis of scientific knowledge by Kant. The latter introduced interesting remarks on the role of the systemic component of regulative principles in scientific theories. To give an example, Galvanis activity is innovative for physiology, that becomes a field of precise and accurate experimental researches of a " Baconian " kind, but is clearly connected with the epistemology here defined as Aristotelian, that is to the attempt to make " sensible ", and thus scientific, the " occult " qualities. Voltas activity instead is more closely connected to the introduction of theoretical terms and to the quantification of these terms. This does not happen along Newtonian-Coulombian lines, but is based on the search for intensive (additive) and extensive (non-additive) factors. The product of these two factors has itself a physical meaning, following an old tradition that started with Oresme, was improved by Leibniz and, after Volta, will be influential in the 19th century in particular on the evolution of the concept of energy.