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.
[Essay II i 5-8] Individual human beings therefore exhibit great differences in their possession of simple ideas, and Locke speculated that other sentient beings—having, for all we know, experiences very different from our own—are likely to form ideas of which we can have no notion at all.
Since simple ideas are acquired only by experience, anything we do not experience is literally inconceivable to us.
[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.
Thus, the crucial feature of ideas for Locke was not what they are but rather what they do, and the epistemic function of an idea is to represent something else.
For since the Things, the Mind contemplates, are none of them, besides it self, present to the Understanding, ’tis necessary that something else, as a Sign or Representation of the thing it considers, should be present to it: And these are Ideas.
[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.
He did commonly refer to them as being “in the Mind,” both when we are conscious of them and when they are stored in memory, he regarded this as no more than a spatial metaphor.
Locke was interested in these immediate objects of perception only because they point beyond themselves.