One of the fundamental theses of Leibniz’s philosophy is that each substance expresses the entire universe.
In order to incorporate this thesis into his general epistemology and philosophy of mind, Leibniz develops his account of “petites perceptions” or “minute perceptions” mentioned briefly in the section on pre-established harmony.
As he puts it in the Preface to the New Essays, “at every moment there is in us an infinity of perceptions, unaccompanied by awareness or reflection; that is, of alterations in the soul itself, of which we are unaware because these impressions are either too minute and too numerous, or else too unvarying, so that they are not sufficiently distinctive on their own.” (A VI vi 53/RB 53) In other words, everything that takes place in the universe really is expressed by each finite mind, but the infinite perceptions present in the mind – from the butterfly’s flight in the Amazonian jungle to the penguin’s waddling in Antarctica – are usually too minute or too indistinct to outweigh, for example, the appearance of this computer screen or the feeling of hunger.
Indeed, this infinity of perceptions is likened by Leibniz to the roar of the sea. “To hear this noise as we do,” Leibniz says, “we must hear the parts which make up this whole, that is the noise of each wave, although each of these little noises makes itself known only when combined confusedly with all the others, and would not be noticed if the wave which made it were by itself.” (A VI vi 54/RB 54) The infinity of petites perceptions is, then, simply epistemological white noise.
For Leibniz, the simplicity and unity of the mind still allows for the multiplicity of perceptions and appetitions. The multiplicity, however, should not only be interpreted as diachronous but also synchronous; that is, the mind despite its simplicity and unity has within it at any time an infinity of different petites perceptions. A human being, in a waking state, is conscious of particular perceptions, but never all. And here we see that Leibniz’s doctrine is important, insofar as it offers a contrast to the Cartesian theory of the mind. According to Leibniz, the mind is always active, for there are always perceptions present to it, even if those perceptions are minute and do not rise to such a level that we are cognizant of them. Thus, even in a deep and dreamless sleep, the mind is active, and perceptions are in the mind.
-Stanford Encyclopedia of Philosophy on Gottfried Wilhelm Leibniz