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.
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 IV xxi 4] Because we do think and must always be thinking about something or other, then, it follows that we actually do possess ideas.
[Essay II i 1] If we want to comprehend the foundations for human knowledge, Locke supposed, it is natural to begin by investigating the origins of its content.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.An adequate genetic account will explain, at least in principle, how human beings acquire the ability to think about anything and everything.Let us then suppose the Mind to be, as we say, white Paper, void of all Characters, without any Ideas; How comes it to be furnished?[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.[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 II ii 1-3] Everything begins, then, with simple ideas of sensation.Most of these are uniquely produced in the mind through the normal operation of just one of the organs of sense.