John Locke, "Of Perception":
Sec. 2: What Perception is, every one will know better by reflecting on what he does himself . . . .
Sec. 3. This is certain, That whatever alterations are made in the Body, if they reach not the Mind; whatever impressions are made on the outward parts, if they are not taken notice of within, there is no Perception. Fire may burn our Bodies, with no other effect, than it does a Billet, unless the motion be continued to the Brain, and there the sense of Heat, or Idea of Pain, be produced in the Mind, wherein consists actual Perception. . . .
Sec. 15. Perception then being the first step and degree towards Knowledge, and the inlet of all the Materials of it, the fewer Senses any Man, as well as any Creature, hath; and the fewer and duller the Impressions are, that are made by them . . . . Perception is the first Operation of all our intellectual Faculties, and the inlet of all Knowledge into our Minds. . . .
John Locke, "Of Retention":
Sec. 2. [R]eflection is the Power to revive again in our Minds those Ideas, which after imprinting have disappeared, or have been as it were laid out of Sight . . . . This is Memory, which is as it were the Store-house of our Ideas. . . . [O]ur Ideas being nothing, but the actual Perceptions in the Mind, which cease to be any thing, when there is no perception of them, this laying up of our Ideas in the Repository of the Memory, signifies no more but this, that the Mind has a Power, in many cases, to revive Perceptions, which it once had, with this additional Perception annexed to them, that it has had them before. And in this Sense it is, that our Ideas are said to be in our Memories, when indeed they are actually no where, but there is an ability in the Mind, when it will, to revive them again; and as it were paint them anew on it self . . . . And thus it is . . . that we are said to have all those Ideas in our Understandings, which though we do not actually contemplate, yet we can bring in sight, and make appear again, and be the Object of our Thoughts, without the help of those sensible Qualities, which first imprinted them there.
from An Essay Concerning Human Understanding (1690).