Empedocles wrote:We would not experience anything if experience were not present in the universe.
To take a look at Leibniz's early form of panpsychism... He seemed to hold that any entity, as it existed independent of outer relations or representation in something else, required its own qualitative occurrences just to have be-ing at all, to have a manner of existing in itself [quote #1 below]. He dismissed a supposed absence of such in most entities (simple monads) as due to their lacking the logical form to properly discern their internal, pre-Established exhibitions of the world (he distinguished between unconscious and conscious perceptions or information, apparently).
Kant, of course, rejected Leibniz's conclusion [quote #2 below] that there were no alternatives as to how things existed in themselves but a phenomenal way, taking an agnostic stance that the intrinsic condition of non-humans (or least non- brained organisms) was unknowable. Perhaps Kant's restriction or influence has lingered on from his critical idealism to today's fixation with materialism / physicalism - although as Strawson has contended, the key source was before Kant:
"Once upon a time, not so long ago, no one thought that there was a mind-body problem. No one thought consciousness was a special mystery and they were right. The sense of difficulty arose only about 400 years ago and for a very specific reason: people began to think they knew what matter was. They thought (very briefly) that matter consisted entirely of grainy particles with various shapes bumping into one another. This was classical contact mechanics, '[the corpuscularian philosophy', and it gave rise to a conundrum. If this is all that matter is, how can it be the basis of or give rise to mind or consciousness?" (from a review of Nicholas Humphrey's book, "Soul Dust")
#1
Gottfried Leibniz ...
Still monads need to have some qualities, otherwise they would not even be existences. [...] We experience in ourselves a state where we remember nothing and where we have no distinct perception, as in periods of fainting, or when we are overcome by a profound, dreamless sleep. In such a state [...we do...] not sensibly differ at all from a simple monad. [...] Nevertheless it does not follow at all that the simple substance is in such a state without perception. This is so because of the reasons given [...] nor on the other hand would it exist without some affection and the affection is nothing else than its perception. When, however, there are a great number of weak perceptions where nothing stands out distinctively, we are stunned; as when one turns around and around in the same direction, a dizziness comes on, which makes him swoon and makes him able to distinguish nothing. Among animals, death can occasion this state for quite a period.
[...] The passing condition which involves and represents a multiplicity in the unity, or in the simple substance, is nothing else than what is called perception. This should be carefully distinguished from apperception or consciousness, as will appear in what follows. In this matter the Cartesians have fallen into a serious error, in that they deny the existence of those perceptions of which we are not conscious. It is this also which has led them to believe that spirits alone are monads and that there are no souls of animals or other entelechies, and it has led them to make the common confusion between a protracted period of unconsciousness and actual death. They have thus adopted the Scholastic error that souls can exist entirely separated from bodies... (Which is curious, since Leibniz's "material bodies" would only be presented as such in perceptions; pre-Established Harmony eliminated the need for windowless monads to literally be organized into spatial systems outside themselves.)
#2
Immanuel Kant ...
Leibniz took the appearances for things-in-themselves [...] The philosophy of Leibniz and Wolff [...] has given a completely wrong direction to all investigations into the nature and origin of our knowledge. [...] It does not merely concern their [logical] form, as being either clear or confused. It concerns their origin and content. It is not that by our sensibility we cannot know the nature of things in themselves in any save a confused fashion; we do not apprehend them in any fashion whatsoever. If our subjective constitution be removed, the represented object, with the qualities which sensible intuition bestows upon it, is nowhere to be found, and cannot possibly be found. For it is this subjective constitution which determines its form as appearance.
[...] the celebrated Leibniz [...] believed that he could obtain knowledge of the inner nature of things by comparing all objects merely with the understanding and with the separated, formal concepts of its thought. [...] He compared all things with each other by means of concepts alone, and naturally found no other differences save those through which the understanding distinguishes its pure concepts from one another. The conditions of sensible intuition, which carry with them their own differences, he did not regard as original, sensibility being for him only a confused mode of representation, and not a separate source of representations. 
