He describes the mind at birth as a blank slate (tabula rasa, although he did not use those actual words) filled later through experience.

The essay was one of the principal sources of empiricism in modern philosophy, and influenced many enlightenment philosophers, such as David Hume and George Berkeley.

In terms of qualities, Locke divides such into primary and secondary, whereby the former give our minds ideas based on sensation and actual experience.

Furthermore, Book II is also a systematic argument for the existence of an intelligent being:Thus, from the consideration of ourselves, and what we infallibly find in our own constitutions, our reason leads us to the knowledge of this certain and evident truth, that there is an eternal, most powerful, and most knowing being; which whether any one will please to call God, it matters not!Locke contends that consciousness is what distinguishes selves, and thus,[10] …in this alone consists personal Identity, i.e. the sameness of rational Being: And as far as this consciousness can be extended backwards to any past Action or Thought, so far reaches the Identity of that Person; it is the same self now it was then; and 'tis by the same self with this present one that now reflects on it, that that Action was done.Book III focuses on words.

Locke writes at the beginning of the fourth chapter ("Of the Reality of Knowledge"):I doubt not but my Reader by this Time may be apt to think that I have been all this while only building a Castle in the Air; and be ready to say to me, To what purpose all this stir?

In 1704, rationalist Gottfried Leibniz wrote a response to Locke's work in the form of a chapter-by-chapter rebuttal, titled the Nouveaux essais sur l'entendement humain (New Essays on Human Understanding).

Leibniz was critical of a number of Locke's views in the Essay, including his rejection of innate ideas; his skepticism about species classification; and the possibility that matter might think (a resolution of the mind–body problem), among other things.

Leibniz thought that Locke's commitment to ideas of reflection in the Essay ultimately made him incapable of escaping the nativist position or being consistent in his empiricist doctrines of the mind's passivity.

He also argues that Locke's conception of material substance was unintelligible, a view which he also later advanced in the Three Dialogues Between Hylas and Philonous.

At the same time, Locke's work provided crucial groundwork for future empiricists such as David Hume.

Likewise, Louisa Capper wrote An Abridgment of Locke's Essay concerning the Human Understanding, published in 1811.