Is it "the policy is a predicable" or "the policy is predicable"?
I'm not even familiar with the word, so maybe I shouldn't say this, but...
You can use either form if you want. I see no difference between predicable and the way we use consumable, for example. Adjectives are often converted into nouns in English, especially in formal/technical/legal contexts.
This ink-jet cartridge is [a] consumable is valid either way.
LATER just for interest, I checked Google books for "policy is [a] predicable". Neither occurred even once, which makes me less ashamed of not knowing the word.
I also ran standard Google web searches on both forms. Disregarding this actual question on EL&U, there are only two instances - and one of those is a reference to the other anyway. Just so you know, it says the Fed's policy is a predicable.
EVEN LATER I can't find any evidence of predicable ever being applied to any type of policies apart from the one reference above. Even though I can imagine an insurance policy being assertable (exercisable, capable of being claimed against?), it's not obvious to me how the Fed's fiscal policy could be thus described. But I'm prepared to believe that's just because I'm not familiar with specialised terminology in these areas (and I suppose OP is).
"Predicable" is an adjective meaning "assertable", and also a noun meaning "an attribute".
In this case, since "the policy is an attribute" makes no sense, the correct form is:
The policy is predicable.
Meaning "the policy is assertable".
The OED gives predicable as:
an adjective meaning ‘That may be predicated (in various senses); capable of being affirmed or asserted’, with the earliest citation dated 1547’;
a noun used chiefly in Aristotelian logic and Kantian philosophy; and
more generally, ‘A thing which may be predicated.’
Two rare derivatives are predicableness and predicably.
The answer to the OP’s question is therefore that both the policy is a predicable and the policy is predicable are possible. I don’t think I’d advise either, though.
jrdoko, can't you simply look it up in a dictionary?
Note that you can always look for functional clues within any sentence or phrase that you find, provided that it is written grammatically. Here, you see the article "a" before 'predicable' in your first phrase. The article 'a' is only used in a noun phrase. That means your word is being used as a noun, whether or not it looks like a noun to you. In English, we are very good at switching words from one of the major parts of speech to another without changing the morphology through a process called conversion: dust (n) becomes dust (v), etc. The nice thing about the second version you provide is that you can attach a prepositional phrase after the adjective : predicable on x basis.... (I think the prep is 'on' here, anyway.) Good luck with your construction.