Prior to the 20th century, there were two dominant positions in Western philosophy about the nature of existence: materialism, and idealism. Materialism holds that the world is made of matter: the stuff of the Earth, molecules and atoms. Idealism holds that the world is made of ideas: the world is precisely our experience of a plenitude of subjective images dancing before our consciousnesses.
Rationality is in support of materialism; all the evidence of science points towards materialism. Hence, with the rise of scientism in the 20th century, materialism has become the overwhelmingly dominant position in Western philosophy. But the materialism/idealism debate more or less lives on, with different vocabulary, in the form of the so-called “mind/body problem,” or the “hard problem of consciousness.”
Rationality speaks of materialism. Our mystical intuitions, on the other hand, speak to us of idealism. They tell us that the existence of the experienced is predicated on the experiencer; that all the multiudinous, dancing phenomena emerge from, and are grounded in, a numinous, immanent Self.
I would like to say that idealism is ultimately true. Materialism shallowly admits only the existence of that which we can see with our eyes. It fails to give the self its due. It notices everything outside, but never stops to look within and see the infinity that lies there. It is the sort of shallow, self-assured, fake wisdom that is typical of skeptics and scholars.
We can formulate the idealist thesis like this: existence is experience. This statement is an equation, unifying two fundamental philosophical concepts. Unifying equations are often important in science. Think of the unifying of light, magnetism, radiation, and electricity into the single concept of electromagnetism. Think of the unification of matter and energy. Similarly, I suggest the unification of existence and experience. (Not that this will constitute a scientific hypothesis.)
This equation mends a hole in our ontology: it tells us the ontological position of consciousness. The mind/body problem asks, in essence, “how does consciousness fit into everything?” It is a deep puzzle that is currently confounding some of our best philosophers. With our equation, the question melts away; consciousness is everything.
But I think that there is another sense in which we do not need to reject matter. Every experience we have, in this material world, is a duality split between matter and consciousness. A consciousness experiences matter. For instance, I (a consciousness) look at a table (matter). And it is always like that. So in this sense we say that materialism and idealism each have half of the picture, and we arrive at an ontology like Cartesian dualism.
We can then extend our mind/matter duality into a trinity: spirit/mind/matter. Spirit is the I, the most subjective. Matter is the “it,” the most objective. Mind is the middle ground in between them, both subjective and objective, the point where “I” and “it” meet.