It's obviously a matter of opinion but I'm actually going to disagree with the previous commenter. His comments are true to an extent in regards to the difficulty of learning Arabic as a non-native speaker in general but don't apply to the specific difficulties of learning Medieval Arabic in particular.

I'd also note that the whole thing about Arabic not marking vowels really is not that big a deal. It's no worse than English being so absurdly non-phonetic. You get over it pretty quickly, and matters more for correct spoken pronunciation than for reading. I also don't think using Arabic dictionaries is particularly difficult, their dictionary structure is actually very well suited to the language, but, yes, does require memorization of 9 or so most common verb forms and the various derived forms of the noun and understanding how the root system works, but that's all like first year Arabic and is pretty much contained in memorizing this chart.

But medieval Arabic, in particular, is not substantially different from modern standard (which is what is taught in virtually all introductory Arabic language courses.) I'm struggling to think of examples where they differ and it's basically down to a very small subset of regularized grammar in modern standard and some words changing definition over time, and that's about it. If you can read modern standard, you can read medieval and classical Arabic. With the help of a good dictionary it's a pretty trivial difference even if, of course, some texts have a very specialized vocabulary that will have you reaching for the dictionary more often than others. edit: oh, and it obviously depends on whether you're working with manuscripts or with a print transcription and whether that print transcription has introduced modern punctuation. The lack of that punctuation can be...annoying, and the script can be, uh, yeah pretty difficult but that's just a feature of working with hand-written manuscripts and is equally challenging in English. That's not really an issue though as there are modern Arabic print editions of virtually all such texts I can imagine you might want to read.

The challenge with medieval texts I would argue isn't the language it's just that the format and logical procession of Medieval philosophical discourse can take some getting used to, as well as the specialized vocabulary of certain genres I mentioned above. But this isn't a linguistic problem as it's not resolved in translation and is actually shared between writers of different languages in the period and is often derived from Greek structures. So if you've ever read Aquinas' Summa it's very similar in structure ("Some say X, others Y say, it would seem X and Y, we reply Z, moreover not X or Y because ABC, and definitely Z because WTF" etc.) to al-Ghazali's Incoherence and other works in or refuting the falsafa tradition. The scan is atrocious but you can see such a translation of Ghazali here: http://www.ghazali.org/works/taf-eng.pdf

Some reference works that might be of interest to you:

The best Arabic reference grammar for the classical language in English remains Wright's Arabic Grammar, usually just called "Wright's Grammar."

While Wehr's Arabic dictionary will usually let you muddle through, Lane's Lexicon is the best classical dictionary in English, based as it is on Lisan al-Arab.

You can find all of these online.

---

All of this is assuming of course that you want to actually learn and read this in the original. For most such texts there are perfectly fine English language translations of, say, Averroes, al-Farabi, al-Ghazali, etc.

Not sure if this is appropriate for this sub, so if this entire thread gets deleted, feel free to message me.

This is actually my area of expertise - I got my degree in linguistics, I know a lot (externally) about Arabic, and I have a close friend who studied it for years, with the goal of reading medieval literature.

The answer is that it's pretty damn hard. The grammar of Arabic is highly foreign to anyone who doesn't already speak a Semitic language, and it requires the memorization of many, many inflectional paradigms, including many that are irregular.

Furthermore, almost all material in Arabic that isn't the Qur'an or children's books just doesn't indicate short vowels, and two of the letters for long vowels are identical to letters for consonants. The Qur'an is written with notoriously difficult language, and children's books, of course, aren't going to prepare you well for the classics.

This isn't a very big deal for people who already speak the language well, but it does make a learner's job a lot more difficult. It's much harder to delve into original language material while learning, because not only does it make it harder to look stuff up in a dictionary, it can also make different grammatical forms of the same root appear identical, and you have to rely on context to decode what is actually meant. And making intuitions on ambiguous words based on context is one of the more advanced skills in any language.