Is the following sentence good/legal/understood English?
Meditation melts the coarse and solidifies the subtle.
If it isn't, how can this be otherwise expressed, in a neat and concise way?
In most European languages, Article + Adjective
constructions can have specific reference; the German nickname for Chancellor Konrad Adenauer, for instance, was Der Alte, which translated literally as 'The Old One'.
While English speakers can and do often delete nouns and pronouns, in context:
generally in English, constructions like that can only have Generic reference, not Specific:
So, the coarse, the subtle, the ridiculous, the sublime, the stupid, the uneducated, all are fine, provided that they don't refer to individual contextually specific things or people, but rather generically, to all such things, without individuation.
What's interesting is whether they have to refer to classes of people, or of things. That's a matter of interpretation, and varies a lot with the adjective and the context.
That's fine, assuming by "the subtle" you mean "things which are subtle".
Compare with the phrase "from the ridiculous to the sublime".
Yes - this is an accepted form of usage. I'm fairly certain that it is used more when speaking in sweeping generalities as in your example "the coarse" and "the subtle."
This is not really less consise than the alternative "coarse thoughts" or "subtle thoughts."
There is at least one situation in which I am pretty sure it is valid, but I don't know what it's called.
I seem to recall from my English lessons that you can use the adjective with a capital to mean the group of people to which this adjective applies. A well-known example being: "The Rich and the Poor".
What you're describing could be a generalization of this construction, although I wouldn't deem it correct.