It is true in the sense that it is a truth for you. If I believed with all my might that the sky was purple then that would be the truth for me even if the sky is blue for you. The world is only what we perceive and how do you know that the sky isn't purple? How do you even know the sky is blue? For you the sky being blue is the truth but that is purely subjective, it is only what you perceive. You could argue that there is scientific proof that the sky is blue because of reflections of the oceans or something (I can't quite remember) but then you are talking about an entirely different form of truth, you are thinking about scientific truth which in itself is only true until another theory is proved to be more correct. This would be a more subjective and universal truth if we fully understood the universe but a scientific truth used to be that the earth was flat, a better theory was then thought of and proved but there is room for change. In both the more scientific and personal truths we are still constantly learning more about the world we live in and this changes the way we see it, thus changing the truths we think we believe. Woops I did not mean to right this much and I hope it makes sense.
I suppose a better way of phrasing that might be: "Isn't there a concrete reality that is the ground for the possibility of our perceptions?
The light down to which the sky refracts does have a measurable frequency that is certain. Our eyes or brains may perceive it different ways, but the reality of the light wave doesn't change depending on who is perceiving it.
I would also say that, in general, having something be "true for you" but not true in reality is a condition we commonly refer to as "being wrong."
That seems like an unnecessary step. Why should we have to say that it is true or "true to them" when we can just say that they believe it to be true? Why do we have to give it more objective credibility than it is in fact due?
If I were to believe the Earth to be flat, my perception would be valid and harmless. However, I would still be wrong. Just because someone chooses to believe or not believe something, and that helps their lives be easier, that doesn't make it true.
I believe we can support people where they are without creating an artificial construct wherein everything everyone believes is right.
Well isn't that the entire justification of 'I think therefore I am'? It is the idea that nothing around us can be truly verified. I have no way of truly knowing any of you exist outside of my own mind, you could be the product of psychosis or you could be real human beings. I only know that I exist because I think inside my own head, and thus I am the starting point for the other perceptions. I know I exist although I cannot truly prove what form I exist in. For example, if I truly believed that gravity did not exist and everything in my world floated, then even if you tried to explain the scientific laws of gravity to me I would not believe you, I would not be able to understand your concept because my perception of the world is that gravity does not exist and things float. That would be a scientific truth in my own mind.
Descartes' "Third Meditation" is indeed a reduction unto self. To prove the existence of God (the reason he wrote this meditation), he has to begin with something that is certain. Notice that he never says that one can never be certain of anything; rather, he acknowledges that from our limited human perspective, the most certain of things is our own existence.
Here's a question: all of the musings, questions, and speculations about whether or not that world outside of yourself is real... are they anything better than pointless?
Pointless questions can truly diminish the quality of your life. Descartes actually examines this as well in the Meditations. It is necessary to trust in order to live. We must trust that the world around us is as we perceive it. Even if it is not so, we can have no peace, nor joy, nor satisfaction without that presumption.
If we can offer reasonable proof based on our perceptions and observation as to the existence and reasonableness of this world, but by perception and observation no one can disprove the existence or reasonableness of this world, what might be the appropriate response?