We use to think of ideas as distinct units of mind which are furthermore also eventually directly communicable. In this context idea is essentially synonymous with thought. That’s why idealists, that is to say people with inclinations to religious, spiritual, or magical thinking, presuppose existence of some kind of cosmic mind similar to the human one as the root of reality.
This position is in stark contrast with empirical solidity of universe which is evidently not subject to the whims of human thought. At least not directly. While human thought can obviously influence universe via translation to energy, it e.g. cannot — not even in an unreliably irreplicable way — move mountains or even mole hills; the thought must be manifested into shovels and (a lot of) manual work for it to be effective.
An argument could be made that a placebo effect does, indeed, demonstrate a direct impact of thought on material body, but this phenomenon never reaches beyond particular individual who is subject to it.
In other words, as thought can make a body move, it can also influence its other functions such as hormonal secretion, etc. which further translates into more visible effects such as heart rate, blood pressure, breathing and so on, including but not limited to the sense of all signals that come from inside one’s body (interoception), which register in human perception as feelings and emotions.
Those undeniable empirical facts then lead to philosophical positions of atheism, materialism, or physicalism, all of which are in strong opposition to all forms of idealism. Or so it seems, because putting them side by side as equivalent alternatives — equally viable but mutually exclusive options — is merely a form of false dichotomy.
For starters, in universe as it is today, information-energy-matter equivalence was demonstrated beyond well-founded doubt. That means that you won’t find information without matter, or, in other words, it means that information is physical, material even — although you can directly transform information to energy, you still cannot violate the second law of thermodynamics.
To explain what information-energy-matter equivalence means in practice, take for example a table. There you have a bunch of data encoded as energy interactions which result in table’s material properties as perceived in relation to the perceiver, who is in turn a conglomerate of phenomena build on the same core principles.
Hence, in a similar fashion, mind is brain. That the brain appears to us as a lump of mucous gray matter while, in fact, it is just a formation of handful of vibrating quantum fields plays no role in this. Brain is its own information that is energy that is matter. Brain in motion (that is, neurons firing — again an observable physical process) is then what registers as mind.
Here it’s important to note that that mind is a human one. Each and every idea is simply a human thought. It’s data contained within energy-matter translated into human symbolic language which is quite specific for each and every individual despite existence of traits (or mind-patterns) common to our entire species.
I would hypothesize that sometime during the Planck epoch an universe of pure information was very briefly present, but quickly “condensed” into the state of energy and subsequently also matter, exactly as it is today.
That would imply that idealistic monism is firmly contained within the observable materialistic monism — or better said, materialistic monism derives from, is further refinement or subset of, idealistic monism — a proposition which information-energy-matter equivalence does, indeed, seem to support.
That does not mean, however, that idealistic monism is still present as a “stand-alone realm”, much less as an alternative to materialistic monism. It just means that the dead people and other “incorporeal entities” that may visit you, or the “memories of your past lives” are in fact (im)personal information presented to you in a form that is understandable within the framework of your particular symbolic vocabulary, and may or may not be actually related to the causal remnants of people and events associated with the aforementioned symbols.
In the end, and as is the case in case of all of these “supernatural events,” it is not the messenger or medium, but the actual message and its gravity what actually matters.
Nevertheless, it also does not mean that materialistic monism is an exhaustive be-all and end-all ontology. After all, it leads to the seemingly unsolvable “hard problem of consciousness”, Cartesian body-mind duality with cop-outs such as qualia (and similar paradoxes), or outright idiocies such as the wild claims about consciousness being a user illusion.
As should be obvious by now, each and every kind of monism is valid only within the already manifested world of phenomena — the Absolute is not one, it is without other. But that you can neither measure nor observe. That “unknowable mystery” can be only known. Thankfully, you are that, so know yourself. I can guarantee you that the rest will follow.