Many of the atheists and materialists like to think they’re the only game in town- that their view is the only rational one, and that every other view is made by the wishful thinking of irrational people. It is because of this that idealism comes out of left field and completely throws them for a loop. It is vastly stranger than anything they can conceive of, and it is actually much more rational as well. It is also arrived at by pure logical deduction and skepticism about one actually being special. Here are some reasons you can give.
1. It is vastly more parsimonious. Some materialists like to think that their view is parsimonious because many scientists hold to it, as well as because it drops dualism’s difficult notion of a soul, but what they don’t realize is that idealism one ups both by never postulating an entire category of things one can never know anything like. How does this work? Everything one knows to exist for certain falls into one of the three categories: thoughts, experiences, the “self” that has both of these things. These are all mental phenomena. Beyond this, anything else requires inference and speculation. Solipsism will go the hard direction and say these are all that exist, and most of the other positions (e.g., substance dualism, materialism) will infer the existence of an entire world outside of these three types of phenomena-which share no properties in common with what one can ever know, or experience. From an empiricist’s standpoint, that makes their views very difficult to truly justify. One can see reason to extend the idea of existence beyond his or her own ego- but no reason to invent an entirely different ontological category that one can never truly experience – and have the audacity to claim that generates our experiences. So idealism is more parsimonious because it only grants that mental phenomena-the only type of phenomena we ever encounter- extends beyond one’s personal self.
2. We can see it producing the phenomena necessary to explain the world for ourselves. While we cannot even see the world outside of mental phenomena that materialists and others infer, we have even more reason to doubt that it can produce experiences, because without being of the same nature as experience, this becomes a mystery. However, we can see mental phenomena- more specifically, our own minds- producing experiences in action in two different circumstances that are common to everyday experience. When thinking of objects, such as pink elephants, we often can internally “see” the objects as we imagine them. We do this all the time when we need to visualize solutions. Likewise, when we sleep, an obfuscated part of our minds generates an entire world for us. One might quibble that this experience is somehow “physical”, but all one has to do is just look around inside these to see that it’s the product of our own thoughts generated by a part of our awareness we don’t typically identity with. So we have two scenarios in which mental phenomena does all the heavy lifting for producing experiences already accessible to us, and there are no testable or recorded instances of a world one can never experience (which is what a mind independent world is) producing the same. (Though some materialists may claim there are)
3. It doesn’t suffer from the interaction problem. Many materialists like to bring up how hallucinatory drugs and other things that disrupt brain activity can influence experiences. Little do they know that this objection only works for dualism, and not other monisms like neutral monism or idealism. Under idealism, the drugs themselves are mental phenomena (as is the brain)- which means that they can obviously affect other mental phenomena, so it’s only as surprising as emotions affecting our thoughts. In dreams, for instance, one can also get hurt, lose cognitive power from injuries, take hallucinatory drugs, etc – but no one will attribute those things to a nonmental world. So why does one attribute the same types of things in waking reality to such, when such is unnecessary? This brings us to our fourth reason.
4. Every objection one can make to idealism, one can make inside of a dream to the idea that they’re in a dream (hint: they always fail). Dreams are miniature versions of idealism being true. Every single reason one can come up with to being in a dream can be applied to idealism. Now, it’s highly unlikely that you’re going to go lucid and gain superpowers like you may in a dream, but there are instances in dreams when such things are also beyond control. Under idealism, the waking state is one in which the other mental phenomena just happen to be much more out of one’s control. Likewise, you might think that reality isn’t nearly as irregular as it is in dreams- and try to claim mind independence(in spite of never having encountered mind independent phenomena in your life and thus not knowing how at all it should behave). However, in your dreams, the irregularities completely slip your mind- as they would do here. The problem is thus: for any objection you make, a person in a dream can make the same objection to idealism being true on that level (which it is), and be as seemingly justified as you are when you make yours. However, every single time that such objections have been tested/testable, the objection to micro-idealism has turned out wrong. Just how confident does someone have to be to keep losing bets?
5. It’s the only actual monism. Yeah, this one kind of had to come last, and for good reason. If you weren’t on edge already, you will certainly be now. What makes this the only monism? The principle that two things that interact must be of the same fundamental nature. For instance, in physics, everything made of matter interacts with other matter, energy, and spacetime, only by virtue of the fact that these things actually are the other things, and are expressible as units of the others, as such. However, other monisms simply lose out when it comes to this (which is why things like materialism give you the Hard Problem of Consciousness- http://consc.net/papers/facing.html). Of course a world that isn’t of the same nature as mind can’t give you mental phenomena without appeals to “emergence” (materialist’s way of saying ex nihilo in a way that’s supposed to be rational), or panpsychism (throwing in the town and just claiming mind is somehow tacked onto everything, without specifying why or how). Some materialists have gone to the point of denying the existence of the mind altogether (cogito ergo – um, no, I don’t). All of this can be avoided simply by recognizing that whatever interacts must be of the same fundamental nature. Our experiences and the thing that experiences have to interact with the external world (otherwise we’d never experience it), so the external world has to be of the same fundamental nature as both of these somehow. That would mean it would be made of similarly mental phenomena (just as matter is made of energy). That would make idealism the only true monism in town. Every other situation just tries to make there be ontologically special distinctions between the nature of one’s mind and the nature of the rest of the world- which is actually a dualist concept (just accept that what you are isn’t that special in relation to the rest of the world). Claiming the world is made of anything else gives you all kinds of problems, like those stated above.
In conclusion, idealism wins all around, has vastly more evidence for it than any other ontology ever can, and is truly monistic.