Imagine there was only one person in the universe, a mad man who thought the sky was green. He says "The sky is green". Is this true? The answer is no. It does has sort of truth though, correspondence with ones beliefs, a personal truth.
That situation is a trick because we the observer also had a presence in the universe. We see it not from the mad man's perspective but our own. We know that the sky is blue, so we can say that the statement is false. What if we also believed that the sky was green? Would the statement then be true?
Imagine that the world was filled with people who all believed the sky was green. One says "The sky is green". Is this true? Again no. Again this has a sort of truth, correspondence with the common consensus, but the statement is not true. Ask yourself; Before Copernicus did the Sun really orbit the Earth just because the authorities insisted it did?
Real truth is when a statement equates with "reality", yet defining reality is a problem for philosophers because it is built from our perceptions, and so truth appears to be an equation between the beliefs about our thoughts and the beliefs about our perceptions. If both sides of the equation involve our beliefs then anything could be called true. What is the solution?
The solution is to assign a mind to the universe. Something is true when our beliefs are the same as the beliefs of the universe. The universe consists of everything that is not us. Like any being, we must assume that the universe has perceptions and beliefs, the perceptions of all things.
In the first example there is only one man in the universe, a mad man who thought the sky to be green. We the observer are also present, but now the universe is present too. There are three beings. The universe believes the sky to be blue so when we believe it, it is true. When the mad man believes it, it is false.
Perfect establishment of the truth is a matter of communication with the universe to establish its beliefs then, and to establish truth we must communicate with the universe as we do with other people to establish their beliefs.
What then is belief? Belief is nothing more than the dictionary definition. The belief of the universe can be considered truth at all times, but the beliefs and thoughts and opinions of the universe must be established by enquiry and reason, in the same way as the beliefs and thoughts and opinions of any person are established. We can never observe from anyone's perspective except our own, and we can never see the universe from its perspective, so knowledge of truth is difficult or impossible, but it can be established to a greater or lesser degree by enquiry.
Let us consider another experiment. Let us assume that the world consists of people without eyes. They all think the sky is green. The people develop a machine to detect the colour of the sky, but it is flawed and speaks that the sky is green. One inhabitant declares "The sky is green". Is this true? No, because the universe believes the sky to be blue. The machine was flawed, so the enquiry into the beliefs of the universe was flawed.
Considering the question "Are green hats more pretty than orange hats?" creates an interesting dilemma. The statement is true if the universe agrees. Prettiness might be an opinion of people, even all people might agree that a tree is more pretty than a building, but would that be true? It depends if the universe agrees, and working out if and how the universe agrees is a matter of enquiry. Prettiness is not a simple argument. There are pros a cons for each object regarding prettiness, and the universe might have mixed opinions on the subject. It is the matter for the enquirer to try to establish the beliefs and opinions of the universe. However, it is quite possible that the universe has no opinion on a particular subject, and it's also possible that the universe does not know the answer to a particular question. Why?
The universe holding total knowledge would only be possible if the universe was infinite in size and so infinite in possibility, but as the universe is defined as that which is not us, then the universe must exclude at least us, and so is not infinite. As such the universe cannot know everything. In particular the universe cannot fully know us.
By the same degree we cannot fully know it. If we knew the universe completely then truth would be when our beliefs equated to our beliefs, a nonsense. If the universe was infinite all things would be true. So then, it essential that enquiry by us is imperfect, and essential that the knowledge of the universe is finite and limited.
Things are additionally complicated by the fact that we are in the universe and part of it, however, essentially some part of our minds must be independent of it because of the arguments outlined above. This begs the question; what is mind?
Mind in this context is a container of knowledge. This means that mind is divisible. A rock contains knowledge of (at least) its shape, cut in half a becomes two rocks each with knowledge. The rock cannot fully know itself. Any mind cannot fully know itself or its parts. Divisible to to smallest particle, an object cannot know itself or be fully knowable, because if so truth could not exist.
The barrier between our complex minds and external reality is essentially present for logic to exist and function.