My friend Eric has passed on to me a copy of your letter. Since it addresses some points which I had been meaning to write about anyway I'm using it as an excuse to get started.
Let me assume that we both have similar ideas when we use words like "existence" or "reality." We mean that there is something which is outside ourselves but of which we are inseparably part, that this something is unique and is common to everyone so that if any two people were to examine it they would find that it has the same properties.
Now people have different subjective impressions of what reality is. I will argue that these impressions can, and should be, compared with objective reality. Where they do not coincide we must assume that the subjective impression is a delusion. No one can "know" that a subjective impression is correct, however strongly they may feel that they do. "Knowledge" means that what one thinks corresponds to some part of objective reality.
If something exists, it must be possible to demonstrate that it does, that is, it must have some observable effect. To use your expression, it must be possible to get it to "make a physical, mechanical dial move." If something does not have any observable effect then it makes no difference whether or not we suppose that it exists. For simplicity then, we might as well say that it doesn't exist. Occam's Razor and all that.
If I say that I have fairies at the bottom of my garden but that they are completely invisible and have no detectable effect on anything, then it is not up to you to disprove the existence of the fairies. You are quite at liberty to assume that they do not exist. It is up to me, as claimant, to produce proof that they do. If I cannot, you may conclude that they do not exist. For "fairies" you may substitute any other "subjective," paranormal or supernatural phenomenon that you wish.
To talk about "scientific proof" is a redundancy unless you mean to imply that only scientists can prove things. In fact, anyone with their head on straight can prove things, if they don't get carried away with the, "I know it's true therefore it is," syndrome. As Robert Heinlein put it in a speech in 1941, "I mean a comparatively simple thing by the scientific method: the ability to look at what goes on around you. Listen to what you hear, observe, note facts, delay your judgement, and make your own predictions. That's all there is, really, to the scientific method: to be able to distinguish facts from non-facts."
I would agree absolutely that, "Our theory about the nature of reality must give way to experience." This is what scientists are doing all the time. Many a beautiful theory has run aground on an awkward fact. But experience must be tempered by method.
To say, for example, that "homeopathy works" implies that there is some verifiable advantage to taking a homeopathic remedy. The only way you can claim this is if you have done double blind tests on a very large number of people. One person's experience doesn't count. Diseases have their ups and downs; if the ups happen to coincide with when you took a homeopathic remedy you may think this indicates cause and effect but it could be just chance. If you took the remedy and felt better this may be a purely subjective effect; feeling better is exactly the effect you expected, so you did.
By experimenting on many people and by not letting the people or their doctors know whether they are taking the remedy or a placebo you can distinguish between a remedy which has a real physiological effect and one which does not. Unfortunately, properly conducted tests show that homeopathic remedies have no pharmacological efficacy. Homeopathy's early successes, you will remember, date from a time, 1810 (which is not yet "over 200 years" ago), when being in the hands of a doctor shortened one's life expectancy. Being treated with a potion containing nothing but water was a considerable improvement over being dosed with a poisonous mercury compound, for example. However, medical treatment has improved since those days, even though a lot of people haven't realized this yet.
Don't get carried away by herbalism's mythology. If the Great Spirit or whoever had really planned the plant world with us in mind we would be able to digest grass. As it is we mainly subsist on fruits and seeds which is terribly wasteful. It happens that some plants contain substances which are valuable foods or medicines but this is happenstance and evolution, not a master plan. A herb may contain something useful but it also contains a lot of unnecessary and sometimes dangerous junk. What science does is figure out which substances are good for us and how to remove the substances which are not. Then it often turns out that it is cheaper to synthesize the useful substance directly than to extract it from the plant. "Natural" products are often an uncertain and dangerous mish-mash even when they do contain something of value.
My quarrel with therapeutic touch is three-fold. Firstly, it has not been demonstrated to work. Secondly, there is no theory about human energy fields which makes any sort of physical sense. Thirdly, if these fields exist and can be manipulated by waving one's hands around, how do we know that the effect on the patient will always be good. No medical treatment yet is both effective and completely free from potentially dangerous side effects.
You started out by saying that there is no scientific proof of, to pick one, beauty. I agree completely but this is because beauty is a projection of a human emotion onto an external object. There is a human emotion, the sense of beauty, which is some kind of chemical and electrical effect in the brain. This does not imply that there is any concrete thing called beauty, any more than objects have Aristotle's "amatory principle" which causes them to fall to Earth.
To say that something is "beautiful" is a shorthand way of saying, "This object tends to arouse the emotion we call a sense of beauty." What we are detecting is a property of the human brain, not a property of the object. If many people agree that a given thing is beautiful this indicates that their brains are wired up more or less the same way, not that there is something special about that object. The objective existence of the state of mind "sense of beauty" could be verified should anyone doubt its existence.
People complain that science takes the wonder or the mystery or even the fun out of things. You refer to "de-humanizing and de-spiritualizing." I say that science doesn't change the character of things, it merely changes what we know about them. If we once thought that something was spiritual and science says it really isn't, that doesn't make the thing any less spiritual than it was, it just means that we were in error before and now we know better. The spirit Boreas used to push the winds around. Are you any the worse off for knowing that winds are caused by pressure differences in the atmosphere caused by the sun's heat? At least you are spared the futility of trying to placate the spirit. Of course, you are at liberty to imagine spirituality if you wish but now you know you are fooling yourself.
In my mind, the new knowledge which science has given us makes things seem even more wonderful than they were before. The 6000-year-old fiat world of the Bible which consisted of the Earth and a few hundred miles of space in a dome above it is a paltry thing compared with a universe which is 15 billion years old, is billions of light-years across, contains perhaps 10^22 stars and goodness knows how many life-bearing planets. And this has evolved all on its own, from a bunch of hydrogen atoms into the wonderful and complex living world we see now. Without science you wouldn't know a thing about it.
One thing at which science has succeeded beyond all expectations is in determining what the physical world is made of. We know that the physical world is made up of a limited number of types of particle. We know that there are no more than four forces which act on these particles. This eliminates a whole slew of unwarranted beliefs. If you admit that the human brain is made of normal matter, you admit that it has no properties which cannot arise from the action of the known forces on the known types of particles.
For example, the only two forces which can be used for long range communication are the electromagnetic force and the gravitational force. Since both of these forces obey the inverse square law, and propagate changes at the speed of light, we can be sure that experiments which claim that telepathy is instantaneous and unaffected by distance cannot be correct. Indeed, since we can make very sensitive detectors for these forces, and they don't detect them outside the brain, we can be sure that telepathy does not exist.
If people have minds or souls in any real sense then these must be material and must communicate with the brain using the forces to which the brain is sensitive. If the brain can be influenced by the mind then so can suitable instruments. If the soul is physical (or non-existent) and if this means that when you die, you die, so be it. You are not one whit better or worse off than anyone else who has ever lived. A desire for continuation doesn't indicate that you, or anyone, does continue, whatever Buddha may have said. We didn't ask to be players in this game but we are stuck with the rules as they are.
It seems to me that someone who accepts that life is not a dress rehearsal is going to achieve more than someone who regards it merely as an intermission in eternity. As for what we can achieve, the only thing an individual can do which really matters is to make a contribution to the group knowledge of the human race. Knowledge is the only thing which is potentially eternal.
As a parting thought, you seem to misunderstand the modern, as opposed to the ancient Greek, meaning of skepticism. You say, "To be totally unbiased, one must neither believe or disbelieve." But this is exactly what the modern skeptic does. The skeptic does not prejudge an issue, does not believe or disbelieve, but waits for the evidence.
The skeptic is not a debunker, the skeptic merely says, like Doubting Thomas, "Show me your evidence, then I'll believe." Since skeptics are also human they may have beliefs which they cherish but, if they are honest, they will let the facts overrule their beliefs. I too am comfortable with uncertainty and ambiguity but I am also comfortable with knowing that there are a great many things which I don't need to bother my head over since they are no more than other people's subjective fantasies.
For more on Epistemology, see an excellent lecture on the subject by Professor Wisdom.