The accepted view, the mainstream social-consensus philosophy, is realism: that the world exists irrespective of the presence of an observer. Materialism or physicalism adds that what truly exists are the enduring material particles, or nowadays the electromagnetic fields of which particles are said to be excitations, and the physical laws that determine their behaviour. H. Sapiens ('the observer') is said to be a product of those same elementary entities, and has only arrived on the scene very recently, in evolutionary and geological time-scales. Absent that observer, the Universe continues on its merry way, much the same as it did prior to evolution. Intelligent beings and the mind are epiphenomena, fortuitous extrusions of a generally mindless process.
My argument against this common-sense view is simply that any judgement about 'what exists' – whether it concerns something near to hand, like the screen you're reading this on, or something distant in time or space, like the early Earth – relies on an implicit perspective. If you imagine or depict an early earth, prior to the advent of h. sapiens, even containing no intelligent life – this depiction still implies a perspective, a point-of-view. And this is because without a perspective, there can be no scale of either distance or duration. It is precisely that scale, that perspective, around which the mind organises its sense of reality. Absent a perspective, no thing is nearer or further, larger or smaller, in fact there can be no things as such, as it is the mind which designates the 'thing–hood' of particulars as specific objects ('this' object as distinct from 'that' object).
I make this point because even philosophers and scientists debating these matters still exhibit confusion about it. I was watching a panel session yesterday which included Bernardo Kastrup, and this was one of the first points that came up (not from BK, of course!) 'How can you say the "observer creates the world" when we know that h. sapiens have only recently evolved? Surely the world didn't come into existence only then!'
The problem implied in this question is the conflation of two different frameworks, namely the objective, naturalist framework, on the one hand, with a metaphysical framework on the other. In other words, it is looking at the question through an implicitly naturalist point of view, which assumes the reality of the objective domain first and foremost, and then wonders how it can accomodate the idea of mind generating the world. But here it is already looking at 'the observer' or 'the mind' as something in the world – in other words, treating the observer as part of the domain of objects. But it cannot reconcile this, because the two accounts are operating at different levels of explanation. The naturalist account starts from the apparently–indisputable fact of perceived reality of objects and subjects, while the idealist account seeks to be critically aware of these purported facts.
The common objection to the idealist account is based precisely on this confusion. 'You're saying "the world doesn't exist" outside your consciousness of it! " My response to that is that the idealist account is not making exactly that claim, because 'non-existence' is just as much a construct as is 'existence' - you have the idea of the world not existing, or of nothing existing at all. This too is the 'imagined non-existence' of the world. But again, all judgements about what exists or does not exist are themselves mental constructs. And you can't imagine or conceive of anything outside that, because any act of imagination and conception are obviously mental activities ('vikalpa' in Buddhist terminology. Notice also the convergence here with Kant's transcendental arguments. )
Compare with a passage from the early Buddhist texts:
Kaccānagotta Sutta wrote:“By & large, Kaccāna, this world is supported by [takes as its object] a polarity, that of existence & non-existence. But when one sees the origination of the world as it has come to be with right discernment, ‘non-existence’ with reference to the world does not occur to one. When one sees the cessation of the world as it has come to be with right discernment, ‘existence’ with reference to the world does not occur to one.
Source
This passage and others like it are said by scholars to be the original impetus for the Madhyamaka school which was to become highly influential in later (Mahāyāna) Buddhism. And from the very outset, the Buddha understood the process of 'world–construction' which the mind cannot help but engage in, and in which most beings (according to him) become entangled.
So that is one aspect of the way I've come to understand idealist philosophy. Still to this day, most people I discuss it with (which is very few people!) will simply dismiss it as solipsism or the idea that 'the world is all in your mind'. They won't have the patience to actually understand the argument.