Locke's main philosophical concern, like many of the philosophers of the seventeenth and eighteenth centuries, was epistemological. Specifically, his questions were about the capabilities of the human mind. His theory of knowledge is of major importance to the empirical philosophy that succeeded the continental rationalism founded by Descartes. The Essay, he maintains in the opening sentences of Book I, is a critical inquiry “into the original, certainty, and extent of human knowledge, together with the grounds and degrees of belief, option and assent.”

He was an original, the first philosopher who set out to explore the limits of knowledge starting from an empirical base. It took David Hume to expose his vulnerable flanks. For David Hume — who alongside Locke appeared like a radical skeptic — thought that Locke was inconsistent in his empirical principles. In particular, he maintained that Locke's ideas of substance, the self, and God cannot be known from experience.

As Locke had maintained that all knowledge begins with sensations, so too did Newton embrace empiricism as the basis for all scientific laws. Take any one of his laws, such as the law of gravitation, and you will find that it describes the observable behavior of natural bodies and justifies your calculations about the future.