Locke’s philosophy of epistemology is the foundation of his philosophy of science. Being a philosopher of the enlightenment his epistemology is metaphysical, though he took large strides to create a philosophical grounding for the experimental science of the day.
And if he is a devotee of the new science, he often appears an uncertain one, recognizing profound difficulties in it. In consequence, Locke’s work is characterized by tensions and nuances…
On the one hand Locke retains an ideal that all knowledge is demonstrative and certain. A view shared with the Aristotelian’s and Cartesian’s. On the other hand, impressed with empirical methods, he defines a distinct kind of knowledge, one inferior to genuine scientific knowledge but appropriate to human sensory capacities.Locke inherited the philosophical conception of scientia which has roots in Aristotle. Scientia proposes that scientific knowledge is certain knowledge of necessary truths, which can in principle be expressed in syllogistic form, the conclusion following from self-evident premises (deductive). Locke develops the concept of scientia by distinguishing 3 separate kinds of knowledge: intuitive, demonstrative, sensitive.
Intuitive knowledge is the most certain because the truth is grasped immediately. There are no intermediate steps, and doubt is impossible because the mind can no more avoid recognizing the truth than the open, functioning eye could avoid seeing light when turned toward the sun (E See IV.ii.1, p. 531). Demonstrative knowledge, though also qualifying as certain, is less so because it involves intermediate steps. We cannot grasp immediately that the three angles of any triangle are equal to two right triangles, and must instead construct the steps of a proof.
Locke’s objects of scientia: real essences and the necessary conditions that flow from them (causality?). Locke then distinguishes between real essences and nominal essences. The real essence is that which makes the thing what it is, and the nominal essence consists in the set of observable qualities we use to classify a thing (may change over time).
We have scientific knowledge of something when we know its real essence and, since its qualities flow from that real essence, when we know the necessary connections between the essence and its other qualities. Geometry serves as an exemplar, as it did for so many of Locke’s predecessors. In knowing what a triangle is, we cannot conceive things being otherwise than that the sum of its three angles equal the sum of two right angles.
Real essences are impossible for the human being to know, due to the constraints of sense data. Therefore the epistemological question of knowledge needs to be addressed. Locke rejects skepticism. Instead he lowers the bar for what defines knowledge, invoking another kind of knowledge, one which is not certain; sensitive knowledge. This is knowledge of effects that have come to our senses, without us determining or developing an understanding of their cause. Sensitive knowledge does not get to understand real essences, instead it knows only those perceived properties from which we construct nominal essences. Experience.
Has contemporary science enabled us to go beyond sensitive knowledge—have discoveries about compounds, elements, and subatomic particles provided us with knowledge of real essences? Much of the force of this question derives, to paraphrase Nicholas Jolley, from the fact that many of those discoveries about matter’s structure were not conceived empirically, but only confirmed empirically; they were initially conceived as possibilities by employing the hypothetico-deductive model, and the predictions deduced from the models were then compared to empirical data (Jolley 2002, p. 69). Yet as Jolley also points out, these commentators may have missed the full import of Locke’s geometric model; in a passage quoted earlier, Locke tells us explicitly that if we knew the real essence of gold, we could deduce its qualities even if gold did not exist.