First of all, let me clarify that I'm talking about non-subjective idealism with respect to the physical world (Kant and Berkeley are two examples) and about indirect realists such as (I believe) most scientists are today.

I'm looking for a list of differences that could be presented to someone who is not familiar with philosophical method and thinking. For those who are philosophically sophisticated, the differences may seem profound, but that's because philosophical thinkers think in metaphysical terms. To them, there is already a clear difference between matter and ideas. But what is that difference?

Take something as simple as a stone. There is a famous "refutation" of idealism by a Dr. Johnson who kicked a cobblestone and said 'I refute it thus'", but that doesn't actually refute any claims that idealists make. Idealists agree that there is really a stone, and if you kick it, it will really scuff your shoe; they just maintain that metaphysically the stone should be analyzed into the sense impressions that it makes.

Similarly, there is an attempt by idealists to refute realism by analyzing the disconnect between our perception of the stone and what is really out there in the world, but that doesn't affect indirect realists who are happy to acknowledge the pivotal role that our perceptual equipment plays in our perception of the stone.

We are left then, with two positions that both agree on the empirical and practical facts, arguing over details that seem a bit esoteric and fanciful. There was even a significant philosophical movement called logical positivism that made this very argument in the early 20th century.

How would you explain the significance of this distinction to someone who doesn't think the base ontological difference matters?

  • If I can think through enough details to turn this into an answer proper, I will: for now, my own sense of these terms is as general ideality (ideality as the order of facts in themselves proceeding from general to particular/abstract to concrete, vs. realism being ordered the other way) and particular ideality (ideals as limits, something like limit sets no less: then procession from limits to subsumed elements vs. from elements towards their limits). Commented Oct 13, 2022 at 0:38
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    Certainly there's (perhaps huge) difference between these two positions though not necessarily reflected in your mundane kicking stone example, for example, the ideality of space and time suggested by Leibniz vs their container-like reality develop to later hole argument and different dynamics such as contemporary relational shape dynamics vs general relativity. Of course on math side you have the famous Cantor's paradises lauded by Hilbert for many to dwell in... Commented Oct 13, 2022 at 4:56
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    The idealist says the real world is written in code, the realist says the code is a message describing physical reality, and the pragmatic takes the rock as a neat looking paper weight while the other two are distracted.
    – user18050
    Commented Oct 13, 2022 at 5:08
  • I read that dualists, idealists, neutral monists are all realists. I don't understand.
    – Starckman
    Commented Jan 31, 2023 at 5:28
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    @starckman, I don't know what you were reading, but in normal philosophical terminology idealists and realist are opposites. Did you mean they are all rationalists? That would make more sense, although it would require an argument. Commented Jan 31, 2023 at 6:09

2 Answers 2


I would suggest you rather use the modern terms Internalism and Externalism. The positions are not necessarily based on ontological but at the very basis epistemological differences:

Externalists can claim that the external world is the ontologically primary world because they claim that we can really know how the external world is (and more often than not fail to provide an explanation for how that is possible).

Internalists claim that since we can only know our internal reality, the picture we make of the world, assuming anything about the external world would be mere speculation since we cannot verify how well our representations represent anything, actually not even if they represent anything (albeit we are logically forced to assume that). Thus, since we do not want to delve into speculative metaphysics, the primary world has to be that of our mind.

Fun fact: in his book The Limits of Realism (2013, Oxford UP), Tim Button shows that both positions are at their hearts logically inconsistent. So we are left with a quest to search for a middle ground.

Physical states vs Qualia: The core philosophical problem here is the hard problem of consciousness. As long as we cannot provide an explanation for how our felt experience can arise out of physical states, there still is a gap between our conscious experience and the outer world. The arguments in favour of upholding internalism are falling apart rapidly though because neuroscience can evoke certain distinct conscious experiences by direct physical manipulation already.


According to one contemporary interpretation of the realism/idealism debate, the core difference between realism and idealism concerns their respective implications about the possibility of unthinkable aspects of reality. Realists allow, in principle, that the world could contain aspects that are unthinkable by beings like us. Idealists, in principle, do not allow the possibility of any truths about the world that outstrip our powers of representation. A statement of this position can be found in Thomas Nagel's The View from Nowhere (Nagel, 1989).

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