In New Essays on the Human Understanding, which was a sort of very long Platonic dialogue critically discussing Locke’s landmark Essay, Leibniz took a fascinating and extremely unexpected approach to defending what he took to be the old doctrine of innate ideas that Locke had begun by rejecting. In so doing, he completely transformed its meaning.
Leibniz describes us as inhabiting (or perhaps floating on the surface of) an immense sea of tiny perceptions below the level of conscious awareness. He says that these microperceptions are always ongoing, even in sleep.
This seems to be the first major anticipation of later notions of the unconscious. Perhaps microperceptions might more accurately be called preconscious, as one might say about the Kantian synthesis of intuition, which could be considered to use Leibnizian microperceptions as part of its material. On the other hand, even the Freudian unconscious has been reinterpreted in an expansive way no longer tied to metaphors of depth and containment, which seems to mitigate the difference. (See also Kantian Intuition.)
One may imagine how unconscious microperception might be explained in terms of Leibniz’s monadology, as always-ongoing perceptions of tiny monads included by our larger monad, in his famous image of monads within monads in series without end.
Microperception in the New Essays seems to be attributed to us as natural beings. This is different from what he says about high-level apperception, which was mainly developed in the very different context of his work Principles of Nature and Grace. There, he attributed apperception to participants in what he distinguished from the realm of nature as the community of spirits subject to grace. In terms of the development being pursued here, that would mean that we have apperception as ethical beings directly concerned with normativity, whereas the hypothesis of microperceptions would belong to biological and psychological explanation that is only normative at a methodological level.