Berkeley contends that humans are only aware of two categories of phenomena… 1) Ideas, and 2) Spirits.
Ideas include Extension, Figure, Motion, Number, and the like. Ideas are not protagonists in the great drama of life; they are always acted upon.
Spirits are active and indivisible substances. They are uncompounded, and because of this, they are not subject to change or decay. This means that Spirits are eternal. A human soul is one type of Spirit.
Berkeley states that “we cannot know the existence of other Spirits otherwise than by their operations or the ideas by them excited in us.” I know other Spirits exist by evidence of their agency in the world, just as those same Spirits only know of me by my own signs of agency.
“We do not see a man,” writes Berkeley […] “but only such a certain collection of Ideas as directs us to think there is a distinct principle of thought and motion, like to ourselves, accompanying and representing it.”
Spirits — which Berkeley appears to consider synonymous with Thinking Beings– have a real existence whether they, themselves, are perceived or not.
On the other hand, anything that is NOT Spirit cannot and does not exist unless and until it is experienced by a Spirit. “The very existence of an Unthinking Being consists in being perceived,” says Berkeley. “There is no senseless, unperceived substance.”
What About, Say, The Inside Of A Plant?
One problem with Berkeley’s interesting supposition is what I call The Problem Of The First Domino… Consider the case of a chain of events which starts in some era or region of the Universe in which no one is around to perceive the first events in the chain. This could be anything from the slipping of an asteroid from its orbit to the formation of the Earth billions of years ago. We can see today the current effects of those ancient causes… an asteroid crashing into a life-filled Earth– but Berkeley’s line of reasoning would have us believe that, since no one perceived those original events, that those original events must never have existed.
Berkeley could make the argument that, by perceiving their effects, we are in a sense perceiving the causes… According to such an argument, the long chain of causation could be viewed as “one thing.” But Berkeley doesn’t take this tack.
Something similar comes up when he mentions the innerworkings of plants, which we cannot observe, but which seem to be occurring and nourishing/growing the plant. What Berkeley assumes in this case– and indeed, for any case in which there is no obvious perceiver– is that there actually must exist a perceiver, and if no other perceiver can be found, than it must be God, Himself. This is also why Berkeley believes that things continue to exist even when we turn our backs on them– God, the ultimate and greatest Spirit, is… always… watching.
I think many people consider this the point where Berkeley’s philosophy falls apart, considering the God-is-watching argument as a sort of cop-out. Be that as it may, it would not only be unfair to Berkeley to therefore dismiss him entirely– tossing out the baby with the bathwater so-to-speak– but we would then cheat ourselves out of the exquisite consideration of so many of his substantial metaphysical cogitations. Anyone who leaves off Berkeley here is missing out on most of what is best and most thought-provoking in his thought. I don’t call him the world’s greatest metaphysical philosopher for nothing.