Our own view of the world is all we have, all we know, and the only thing that reminds us we exist at all. This appears to me as an obvious fact, but one which I'm always willing to explore other possibilities. I must go on alone because there is no other way to go on.
All that we experience, we experience alone: we are conceived in darkness, alone, and we die into darkness, alone. I shall say that all we have is our own thought, our self, to keep us company.
What else can we believe?
If we start with the simple premise "I think therefore I am", we can begin to address some questions about what is real, what is unreal, and whether there is anything tangible enough to warrant belief.
Belief is accepting something as truth for which you cannot offer proof. By nature, it deals in the realm of the mind: thoughts, opinions, faith, and so on. Proof may vary depending on the individual: do you need physical, sensory proof in order to believe something? Do you find rational proof, logic, to be sufficient?
Since I lean towards the idea that the contents of my mind are my reality, nothing more or less, then I must also accept that the rational, logical systems I use to explore said contents are also reality (to me). Systems of thought analysis are self-checking, using symbolic expression to make sure everything is consistent. Logic does not in any way guarantee that something will prove to be true, only that it will prove to be consistent (logical)--logic may not be practical or suitable to every situation!
I am alone in my mind.
This is self-evident, an observation gleaned from basic applications of reason and logic. I am also aware of information acquired from some environment. It may be that this environment of which I form a picture is just a projection of thought, a dream, or some other mental delusion. Does this make it any less real from the perspective of the observer? I would think not; trying to convince someone they are delusion when they clearly have belief hasn't worked out too well for mankind.
I must then believe there is something I am conscious of, be it external realities or my own thoughts. I can, at the very least, say so because "I think therefore I am" means these things I think about are also existing in so far as "I am". Other people have similar thoughts, often in perfect agreement with my own. Perhaps this means these other people, other minds, are in agreement because they are projections?
Understanding that what you think about, the content of your mind, is indicative of your existence, is also acknowledging that there is no difference between the object of your thought, the mind that thinks it, and the thought itself inside your mind. To separate these things with language does nothing to dispel the bare fact that it is just a thought and has no existence outside your awareness of it.
So why believe?
When do we begin to accept these thoughts as both real and unreal? Real in that they are apparent to our conscious mind; unreal in that they do not originate with us, saying our thoughts come from something that is not us!
While that may be an absurd reduction, it demonstrates that without us thinking about something, there is no way for us to assert the reality of that something. Existence requires thought and thought exists in the mind alone; each thought part of a self-contained system of reality known as a personality, psyche, etc.
Why believe if it implies we simply believe our own existence? What happens when we believe something outside of ourselves? Do we give our attention, and thus our thoughts, to something else?