I have not been reading philosophy seriously for very long but I have come across George Berkeley, who said that material things are simply ideas in our minds. Therefore if we are not there to perceive the idea of the bird or tree, do they cease to exist? Also are there any faults in thinking in this way? Surely there are. Lets say you had just seen a bird for the first time, and it just so happened that it would let you touch it. It feels delicate, and soft due to its feathers, and then we get an idea and perception of what a bird is based on its properties. So, if we have got this idea from an actual material bird, then surely it is something physical that exists outside of our minds?
First, some terminology: the view that material objects exist independently of anyone perceiving them is metaphysical realism; the view that denies metaphysical realism is called metaphysical anti-realism. Berkeley is a metaphysical anti-realist in this sense.
You ask: what's wrong with anti-realism? Well, it's tough to say without some motivation for the view. So here's a Berkeleyian argument for metaphysical anti-realism:
- Material objects are perceived.
- What is perceived are ideas in the mind.
- So, material objects are ideas in the mind.
Obviously, premise 1 looks pretty good. No one wants to deny that material objects are perceived. And 3 follows logically from 1 and 2, so 3 looks pretty good too. That leaves premise 2. The metaphysical realist should deny premise 2. One way to do so is to hold a version of direct realism. Direct realism holds that material objects are perceived directly, not mediately by ideas.
It's also worth pointing out that your response
So, if we have got this idea from an actual material bird, then surely it is something physical that exists outside of our minds?
begs the question against the anti-realist. The anti-realist denies that material objects exist outside of our minds. Your response assumes that to get the idea of the bird requires that the bird exist outside of your mind. But that's just what the anti-realist denies.
Further reading: https://plato.stanford.edu/entries/realism-sem-challenge/
The world is as you perceive it, and this idea happened to bring me back to a quote from a book series called The Chronicles of Amber, and how they'd asked a variation of this same question, Corwin of Amber and his musings on solipsism: https://cailcorishev.wordpress.com/2014/11/26/corwin-on-solipsism/
It's called spiritualism (philosophy) and in this case there is only two solutions.
Either everything is spiritual (we live in the consciousness of something bigger) and even if the outside world is "fake" you are a part of that "emptiness" so the nature of the reality is just spiritual.
or we only "exist" by the "inside" means that when you look at the tree you make it real. But here the trick you are not the only consciousness here, animals, and if you go further plants maybe the matter it self is looking to things so they make it real.
So in a case you are just a mirror of the emptiness and the other case every things is looking at everything and everyone make everything and everyone exist. At this moment you are not the mirror anymore but the reflection, the "image" of how the world (or you) see itself.