Oh Oo, Well I'd kind of have to disagree then. He's not that much opposite to a scientist. He still uses basic science to create weapons using crafting skills. And he's not very magical because he can't see into the future. Though I'd call him a Sage if I had to because he's more of a kind old person who wants to protect everything and is spiritual. Unless by Mystic, you ment Spiritual too, because that's what he pretty much is.
He is actually very much the opposite, or at least appears to be. Crafting of weapons is an art, a craft, but not a science. Belthesar's things are machines: the Wings of Time, the BlackBird, Chronopolis, even his whole plan, 'Project Kid', was likely based upon scientific principles. He is called the Guru of Reason, after all. Reason, logic, these are the things of science. As I've said before, he is Aristotilian, and it is Aristotle that sets the groundwork for what later develops into our sciences: we view the truth of things as existing in what we can see and sense of them.
Melchior is, again as I have mentioned before, of another, a Platonic, tradition. His title of Life appears to hint at this as well. Life being a more supernatural force, if it were - or at least in so far as spirits and the soul is concerned. I'd wager that Melchior cares less about how things actually work and look, than what they mean, and would hold the truth of them to lie behind in a spiritual world, rather than our world seen to the senses. You use the word spiritual for him, in fact, which is precisely what I meant by mystic. But one cannot conclude that he is not very magical because he lacks extreme foresight. After all, if any in Zeal had had sufficient foresight, they would have sought to stay the day of ruin that was swiftly approaching in the years nearly before the destruction of that kingdom. Or, rather, it may be seen that he DID have foresight, for who other but the Three endeavoured to counter the purposes of the Queen? Schala only, and she either knew of this terrible doom herself through prophetic whisperings, or else learned about it from the masters themselves. But whether or not he possessed foresight is irrelevant: prophecy and seer-craft would likely have been the province of Gaspar. Melchior, through his mystical (or if you wish, spiritual means) forged weaponry - I would hold him to have been the court smith and philosopher of Zeal. Gaspar was the court prophet. And Belthesar the court wise-man in the things of science. However, one must remember that the old form of philosopher is something different than most think it to be today. Philosophy simply means the 'love of wisdom', and in ancient days a philosopher not only theorised on the meaning of life, but upon the nature and ways of the world - they were the scientists until some hundred years ago, when philosophy and science took its seperate courses. So Melchior may have indeed known and used a little science, I concede it, but at least in the forging of the Masamune, his greatest work, I consider him to work not through any scientific means, but merely mystical ones.