Philosophical idealism briefly-
Philosophical idealism has its origins in epistemology- the theory of knowledge, rather than in political philosophy about an ideal state as some believe. Platonic realism in the Republic does have a theory of an ideal state yet the concept of the realm of forms, elaborated more fully and perhaps accurately by the Neo-Platonist named Plotinus in the 3rd century C.E. His work named The Enneads is brilliant yet differs substantially from theories about ideas, perception and reality as one discovers in the works of philosophers beginning with René Descartes who wrote in the 17th century. Plotinus was a philosophical-religious mystic considered heretical by the early church for believing he could experience God directly in heyschasm. Notably Augustine read Plotinus before converting to Christianity. Plotinus’ description of the One and the Intelligence emanating a realm of forms is a remarkable inference.
The realm of forms is an intuited metaphysical account of classes of objects, differentiations of objects and forms, classes and genres, and how they exist. One might say that there are perfect ideas of the form of every possible object or structure that exists in some way in the mind of God and everything that exists in the material universe is a copy or actualization of the perfect form. With temporal objects being subject to destruction over time it is reasonable to infer that a perfect form is eternal (if one accepts the existence of forms and ideas of forms somewhere not quite in this Universe.
Before moving on to the school idealism that began with Descartes. I should point out that the Universe as discovered by physicists actually does have forms at the quantum level that energy must necessarily take to exist creating a certain resonance with Plato’s theory spanning two and a half thousand years.
Idealism is actually, originally ideaism. Reading Descartes Meditations on First Philosophy published in 1641 one can clearly see that it is about ideas that are experienced by the mind, and the way they relate to knowledge. The intense introspection or self-examination of thought considers the problem of the certainty or lack of in it trusting one’s own thought. Maybe from that work the Irish philosopher Bishop Berkeley was inspired to develop the field of idealism further.
Berkeley read John Locke’s works concerning perception and as far as I know created a synthetic advance on Locke and Descartes in finding ideas and perceptions somewhat phenomenal and things-in-themselves. Thought and perception become plainly subjective, including reality to a certain extent. One’s own mind has some challenges in verifying what exists externally and what does not, since all perceptions encounter or present within one’s mind. Today from common knowledge of physics one knows that particle-waves are interpreted through cognitive channels for humans that perceive ‘percepts’.
Berkeley wrote that;
“I do not argue against the existence of any one thing that we can apprehend, either by sense or reflection. That the things I see with mine eyes and touch with my hands do exist, really exist, I make not the least question. The only thing whose existence we deny, is that which philosophers call matter or corporeal substance. And in doing of this, there is no damage done to the rest of mankind, who, I dare say, will never miss it.”, Principles #35
The Universe does exist within a Higgs field apparently, and matter is a secondary quality of energy slowed down to sub-light speed and flat two-dimensional form to three dimensions-hence allowing matter to exist, so I would not become too technical about claiming matter and corporeal substances are this or that myself, instead like many others I might describe matter as best I can in relation to the way the sub-atomic particles and forces of the Universe seem to my subjective experience. Certainly, one may venture an opinion that the entirety of fundamental quanta is funded by the will of Spirit (of God) as Berkeley apparently believed.
Immanuel Kant follows Descartes, Locke and Berkeley so it is easy to see the development and continued work in the field of idealism of the German 18th century philosopher who write the Critique of Pure Reason and other works including the summary version named A Prolegomena to any Future Metaphysics.
Kant investigated perception, cognition, reason and judgments and necessarily, ideas. His theory of judgment was a renewal of modern philosophy and very useful; https://plato.stanford.edu/entries/kant-judgment/
The page above explains two basic fields of theories of judgment- psychologistic and Platonic. One may like Kant’s classifications of the a posteriori, the a priori and the synthetic a priori for a practical way of sorting out what one is thinking and consider how and what kind of knowledge it is. Analytic and synthetic judgments are useful differentiations.
The one may leap forward past G.W.F. Hegel and other German 19th century idealism for combining metaphysics, history, teleology and other concerns that can be somewhat like synthetic evolutionary or teleological tomes, and get on to the 20th century philosophers like W.V.O. Quine (Word and Object) and P.F. Strawson (Individuals) who made a more technical examination of the nature of epistemology in relations to the mind’s ideas and percepts. Those are brilliantly informative works and brings the reader up nearly to the present with analytic philosophy an idealism.
Jean Paul Sartre should be mentioned in the progression. Sartre actually described himself as writing in the tradition of French rationalism, yet one may easily say that he was also qualified as an idealist philosopher who considered subjective thought, experience and knowledge in a nearly Berkeleyian paradigm. Plainly he wrote in the tradition of Descartes. Being and Nothingness is a full exposition of the subjectivity of subjective perception and experience.