Chappell, Vere. "Locke's Theory of Ideas." The Cambridge Companion to Locke. Ed. Vere Chappell, Cambridge, England: Cambridge University Press, 1994.

Excerpt:

“Ideas play a large role in Locke’s philosophy. In Locke’s view, everything existing or occurring in the mind either is or includes an idea; and all human knowledge both starts from and is founded on ideas. The very word “idea” appears more frequently in the Essay Concerning Human Understanding than any other noun; its occurrences outnumber even those of such common words as “he,” “have,” and “for.” Locke’s ideas have, however, perplexed readers and provoked critics from the time of the Essay’s first publication.”

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