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Horst Steinke


in no way contributes to constituting the essence of Nitre. I shall

consider it as the dregs of Nitre […].

Spinoza’s response can be, and needs to be, dissected at vari-

ous levels of analysis. First, one notes the contrast between

Boyle’s epistemic approach, which relies on experiments provid-

ing the basis for inferences (constituting the first


and second

kinds of knowledge, respectively), and Spinoza’s method of

«posit[ing] […] that which is […] obvious», thus independently

of experimental evidence. It consists of «motion» and «rest»,

which are core «common notions», and as such part of the third

kind of knowledge, «manifest enough» by virtue of being «intui-

tively» known. Another level of analysis concerns Boyle’s claim

of “heterogeneity” of the chemical compounds he produced

(without realizing yet the full scope of the chemistry involved)

vs. Spinoza’s contention that the apparent heterogeneity of nitre

was just due to «impurities», thus not disproving its homogenei-



, which he termed «the essence of Nitre»


. These brief in-

troductory statements of Spinoza already lay bare his conflict


with Boyle on his most central commitments: the epistemic sys-

tem, on the one hand, and the very content of his philosophy, on

the other hand, revolving around substance, attributes/essences,

and modes.

In the latter part of

Letter 6

, the subject changes to “fluidity”,

and here, also, Spinoza responds to Boyle from the vantage point

of his epistemic framework (nomenclature added):

In my view, [1


kind of knowledge:] notions which derive from popu-

lar usage, or which explicate Nature not as it is in itself but as it is re-

lated to human senses, should certainly not be regarded as [2



concepts of the highest generality, nor should they be mixed (not to

say confused) with [3


kind:] notions that are pure and which explicate

Nature as it is in itself. Of the latter kind are motion, rest, and their

laws […].