In the second book of the Essay, Locke introduces ideas as the “atoms” of mental life. Exhilarated by his, as he thinks, comprehensive trouncing of the theory of innate propositions and ideas, he declares his empiricist credo that all ideas come from experience, either sensation or reflection. In a very upbeat spirit he turns almost immediately to address the Cartesian claim that the mind is always thinking. He offers the arguments summarized in the previous chapter, each of which flows from his conviction that “thinking consists in being conscious that one thinks” (II.i.19, 115). He also defines “thinking” as “that sort of operation of the Mind about its Ideas, wherein the Mind is active; where it with some degree of voluntary attention, considers any thing” (II.ix.1, 143). And he defines “idea” as “whatsoever is the Object of the Understanding when a Man thinks” (I.i.8, 47). If this is what thinking and ideas are, how could we possibly think or have ideas without being conscious of that very activity?

Transparency, salience, memory and being in the mind

Before very long, however, he begins to realize that this position is certainly not tenable as it stands. It is not that he hesitates about the doctrine of perpetual thought. He remains convinced that there are periods of deep sleep when I am not conscious of being engaged in any mental activity and that I cannot be actively thinking (entertaining any ideas) without being conscious to some degree that I am doing so.