Then, by further experience, they acquire a supply of sensory ideas from which they can abstract, learning to distinguish among familiar things.Only later do they attend to their reflective experience of mental operations in order to acquire ideas of reflection.Whence comes it by that vast store, which the busy and boundless Fancy of Man has painted on it, with an almost endless variety?
Thus, in Book II of the Essay, Locke embarked on an extended effort to show where we get all of the ideas that we do so obviously possess.[Essay II i 21-24] Since we come to have ideas only by means of our own experience, Locke supposed, any interruption of this normal process could prevent us from having them.Having defective organs of sense, artificially restricting experience, or inattentively observing what we have can all limit our possession of mental contents.[Essay II xi 17] Locke had already argued at length that ideas are not innately imprinted on the human mind.Observing children reveals that their capacity to think develops only gradually, as its necessary components are acquired one by one.We acquire ideas of sensation through the causal operation of external objects on our sensory organs, and ideas of reflection through the “internal Sense” that is awareness of our own intellectual operations.As the rest of Book II is designed to show, these two sources provide us with all of the ideas we can ever have.Locke used the word “idea” for the most basic unit of human thought, subsuming under this term every kind of mental content from concrete sensory impressions to abstract intellectual concepts.Explicitly disavowing the technical terms employed by other philosophical traditions, he preferred simply to define the idea as “whatsover is the Object of the Understanding when a Man thinks.” [Essay I i 8] Locke worried little about the ontological status of ideas.To this I answer, in one word, From Experience: In that, all our Knowledge is founded; and from that it ultimately derives it self.[Essay II i 2] The human mind is like a camera obscura for Locke, a darkened room into which bright pictures of what lies outside must be conveyed.