There has always been the problem of knowledge. According to Empiricism, knowledge is obtained by experience, by observing the environment. However, with Idealism, all knowledge is internal. Both of these ideas have problems. For example, I can't point to yesterday because it doesn't exist in the environment. On the other hand, since we were born all of our learning has been by observation. So, it doesn't seem that either Idealism or Empiricism gives a complete description.
There are hybrid versions of these. For example, the Presuppositionalism argued by Sye Ten Bruggencate suggests that all knowledge is contained within the mind of God. Notice that this is nearly identical to Idealism except it shifts the focus from ourselves to God, apparently for the sake of humility. What is most interesting about this concept is the brazen attempt to create an unassailable, philosophical dogma. You can view that attempt
here.
Some of the shift with Presuppositionalism might not be obvious but this is related to framing. I'm sure most people have watched a movie or seen a photograph or painting and considered what was just outside of frame. Naturally on a movie set there is probably nothing outside of frame that would be related to the scene and there is no way to tell what the artist did not imagine outside the bounds of the painting. For a photograph though, unless it was staged, there should be more if you just had a wider field of view or took a series of photos. This is the same problem we have with knowledge. We aren't aware of conversations outside of our hearing or images behind a hill or people we've never met. However, decisions are based on knowledge. What if the answer to a problem was something we've never heard of? That's unfortunate if you are trying to create something artistic or a new piece of technology, but according to most religious doctrines, ethical decisions can be held against you. The idea that you could be judged because of something unknown to you is unsettling but leads to contradiction if you make allowances based on group exclusion.
In other words, various groups in the Bible were judged to be morally worse than the favored groups like Jews. So, God could kill everyone else in a flood or destroy a couple of cities or simply have everyone murdered by a victorious army. But as the doctrine shifts to Christianity, the group becomes more inclusive, everyone could be a Christian. So, theoretically you could be judged simply on the basis choosing not to be Christian. And if we make the claim that the Bible contains all the ethical instruction anyone might need then we should have a self-contained doctrine. Except...what if you've never of Christianity? Or what if you've heard of Christianity but chose not to investigate further? Or what if you only read the first chapter of the Bible but nothing more? Could you then be judged even if you didn't have enough information to make the right choice? The Presuppositional argument seeks to bypass these problems by claiming that everyone has some kind of telepathic link to God. However, this actually brings the focus back to Idealism because, once again, the source of knowledge is innate.
I suppose it might surprise someone that a theological dogma would be related to Idealism. However, many of these concepts are the same. If you were a solitary mind that existed and reality was some process of your mind then how would you be different from an all powerful god that existed and wanted to create the universe? However, the idea that either we could be God or that God wouldn't be all powerful can be unsettling to some. This might then result in an avoidance of the question rather than working it through to an answer.
The frame problem still exists even outside of ethics. In Idealism you have the question of whether anything exists outside of your knowledge. If it does exist then is it as real as you? If it is real then how can all knowledge be innate? There are some options I can think of. You could assert that nothing exists outside of your knowledge. However, that brings up an obvious question anytime you find out something you didn't know. You could I suppose argue that things simply come into existence when your attention shifts to them much like the way the library books filled in when the contents were described in
Pleasantville. However, for anything to exist that you didn't already know would still seem to suggest some kind of agency external to you. Perhaps you could admit that other minds like yours also exist. However, this would allow for knowledge outside of yourself so this too would not really fit. Because of these difficulties, sometimes you see reductions of Idealism where only a portion like wisdom or ethics is claimed to be internal and other facts to be of lesser (or no) importance. However, even a subset tends to be refutable with similar arguments.
You could perhaps assert that while something external might exist, reality is based on your experience. In other words, you could claim that reality is composed of your mental models, classifications, and memories rather than the external things themselves. This can be bolstered by the assertion that external things cannot be directly experienced but only sensed. Let's try that with an example. Suppose I assert that music on a page is not real, only the performance is real. The performance would seem to fit with the idea of personal experience, we sense the music and it becomes real. However, the music on the page is an abstraction much like our mental representations. So, if an abstraction in our mind is real then why wouldn't an abstraction on paper be equally real? If we keep pushing this concept we'll eventually arrive at the idea that our reality is personal to us. But this then also leads to the conclusion that our reality is not different from the realities of other people. This carries the problem of limitations. We can't lift what is too heavy; we can't reach what is too high; and we can't grasp what is too complex. Rather than all knowledge being innate, our knowledge becomes simply the part that fits within our own boundaries. And we can see that those boundaries can be change. If a hill blocks our view we could walk to the top. If something is out of reach we could jump or get a step ladder. If something is too heavy it might not be after some exercise. And, whatever we can't understand today might be grasped tomorrow. This then moves away from Idealism towards Empiricism.
We can also approach this question from other perspectives. What if the effect of something can be detected but the item itself is unknown or undetectable? This has been the case in the past with things like viruses, DNA, radiation, and a whole host of subatomic particles. Eventually we were able to detect them. I suppose it is possible that things could exist like perhaps dark matter, dark energy, or other dimensions that might not be detectable except for their effect. However, just as in the past we normally tend to treat such items as part of the natural world when their effects are consistent even if the exact nature of the cause is unknown. In other words, even if it were impossible to know what dark matter was it wouldn't result in any change to scientific philosophy.
Finally one of the puzzling aspects of a rejection of materialism is that it seems that it can be for two opposing reasons, either wanting to be lesser or wanting to be more. For example, it is not uncommon to hear people assert that humans are not mere rocks or not mere animals. Clearly this is a desire to be more. However, when people assert that ethics cannot be a human concept there seems to be a desire to be less. Likewise, those who practice various forms of self-restraint such as abstinence from sex, not eating meat, avoidance of wealth, or giving up other comforts also seem to be trying to exclude some portion of being human. One does have to wonder to what extent a rejection of materialism is a rejection of self.
