[Essay II i 2] The human mind is like a camera obscura for Locke...

[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. [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.

No individual idea is invariably present in every human being, as one would expect of an innate feature of human nature, and even if there were such cases, they could result from a universally-shared experience. Everything that occurs to us either arrives directly through experience, or is remembered from some previous experience, or has been manufactured from the raw materials provided solely by experience.

[Essay I iv] From the outset of the project, then, Locke took the empiricist stance that the content of all human knowledge is ultimately derived from experience. We can only think about things we're acquainted with in one or the other of two distinct ways: Our Observation employ'd either about external, sensible Objects; or about the internal Operations of our Minds, perceived and reflected on by our selves, is that, which supplies our Understandings with all the materials of thinking.

These two are the Fountains of Knowledge, from whence all the Ideas we have, or can naturally have, do spring. [Essay II i 2] Notice that Locke distinguished sensation and reflection by reference to their objects. 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. [Essay II i 3-5] The acquisition of ideas is a gradual process, of course. Newborn infants, Locke supposed, are first aware of the vivid experiences of their own hunger or pain. 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. [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.