One of Berkeley’s central metaphysical theses is that spirit and perceivables differ toto caelo from one another — perceivables (“Ideas”) are passive, inert; they’re what appears. But to appear, Berkeley notes, is a dative relation — every appearance is an appearance to. To what? Spirit, Berkeley says, whose property is to be active, even if only in receiving ideas. The difficulty with this is that, by definition, whatever one gets in mind is an Idea, which differs toto caelo from Spirit; so, we can’t get Spirit in mind. To objectify the knower itself is simply to form an image of it; when you look for the (transcendental) self, all you find is its shadow, the empirical self. (To use the Kantian distinction.)
Nonetheless, Berkeley takes it that we can know Spirit — slantwise, as it were. We can form what he calls a “notion” of Spirit, reflexively, through the very act of knowing and willing itself. It’s hard to beat Roger Scruton’s description of what this kind of knowing is like, though he would demur from the metaphysical conclusions Berkeley seeks to draw from it: “I know that I am a single and unified subject of experience…I know this on no basis, without having to carry out any kind of check, and, indeed, without the use of criteria of any kind – this is what is (or what ought to be) meant by the term ‘transcendental.’ The unity of the self-conscious subject is not the conclusion of any inquiry, but the presupposition of all inquiries. The unity of consciousness ‘transcends’ all argument since it is the premise w/out which argument makes no sense” (The Soul of Nature 72).
But here’s a tricky question: if Berkeley takes it that possessing a location either in space or time is a property only of Ideas, including the constellations of Ideas that constitute human bodies (he does, more or less), and if possessing a distinct location in (or pathway through) space-time is the principle of individuation for finite substances, then doesn’t it follow that Spirit can’t properly be individuated at all?
Here’s how Schopenhauer, a great student and critic of Berkeley, put the question: The will “is free from all multiplicity, although its manifestations in time and space are innumerable. It is itself one, though not in the sense in which an object is one, for the unity of an object can only be known in opposition to a possible multiplicity; nor yet in the sense in which a concept is one, for the unity of a concept originates only in abstraction from a multiplicity; but it is one as that which lies outside time and space, the principium individuationis, i.e., the possibility of multiplicity” (The World as Will and Idea, §22, p. 146).
Schopenhauer, though he argues with some plausibility that the above position is already implicit in the “Transcendental Aesthetic” of Kant’s first Critique, was also among the first European philosophers to be influenced by Indian philosophy, in particular by the Upanishads as interpreted by the Advaita (Non-dualist) Vedanta. The Advaitins had been making more or less his same point for about 1000 years. Here’s a representative quotation from the Drg Drsya Viveka (Investigation of the Perceived and Perceiver), quoted by Anatanand Rambachan: “The form is perceived and the eye is the perceiver. It (eye) is perceived and the mind is the perceiver. The mind with its modifications is perceived and the Witness (the self) is the perceiver. But it (the Witness) is not perceived” (quoted in Advaita Worldview 37). Krishna puts the point evocatively in the Bhagavad Gita: “The body is called a field, Arjuna; the one who knows it is called the Knower of the field…I am the Knower of the field in everyone” (13.1-2).
Berkeley, of course, isn’t going to be converted just yet. Like Aquinas facing down the Averroist thesis that all of humanity shares a single agent intellect (just like, in fact), Berkeley will have common sense objections, to start: “If there is one intellect, no matter how diverse may be all those things of which the intellect makes use as instruments, in no way is it possible to say that Socrates and Plato are otherwise than one understanding man” (ST 1.76.2). If anything is obvious, surely it’s that whatever constitutes me as a knowing and willing unity isn’t identical with what constitutes you as a knowing and willing unity. Or, at least: even if whatever constitutes both me and you as knowing and willing unities is in fact one and the same, our identity with it, or non-duality from it, can’t require our identity with one another.
The big question here, I suppose, which Berkeley never even attempts to answer so far as I’ve seen, is this: How could God — the Infinite Spirit who creates all that isn’t he — create a finite spirit? Berkeley has an account, drawn in terms of God’s speech, for what it is for the LORD to create Ideas, but he never discusses what it is for the LORD to create another perceiver. The Advaitin and Schopenhauer make the interesting point that, if spirits properly speaking transcend spatio-temporal determination (being the condition for such determination), then it’s hard to see how they can be distinguished from one another or even from God (or Brahman).
Two desiderata for a properly fleshed-out Berkeleyan metaphysics, then, have to include 1) an account of the creation of finite spirits, and 2) an account of the identification of finite spirits with bodies.