Rishon Rishon just posted a table comparing the words in my previous post to Hebrew. Most of them are correct; mo`ed and g'vul are not cognate, and SaH and loa` I'm not sure about. However, one is particularly interesting: ha- = 'al- "the". You often find this seeming cognate cited in works on Semitic: after all, ha- induces gemination of a subsequent non-guttural consonant - suggesting a lost consonant in the prefix assimilating to the subsequent letter - and Hebrew h- occasionally seems to soften to '- in Arabic (the causative measure hiph`iil corresponds to 'af`ala, for example.) Trouble is, the only letter in Hebrew that regularly assimilates to a following consonant is n (although l does admittedly assimilate in the verb laqaH), and the Safaitic inscriptions seem to reveal an early pre-Islamic northern dialect of Arabic which did have a definite article h- (hn- before gutturals, thus hn'lt for Al-Lat.) So are there any other possibilities?
I think so. ha- corresponds pretty well to Arabic haa "here is", which is also the obligatory prefix to the demonstrative "this" (haadhaa, haadhihi, haa'ula'). Compare also Syriac haanaa "this (m. sg.)" - which appears to reveal an added n which could explain the Hebrew doubling (and the Safaitic form) nicely. Conversely, 'al- corresponds well to Hebrew 'elleh, Arabic haa-'ulaa', Syriac haaleyn "these". The vowel doesn't correspond exactly, but then it doesn't in the certain cognate 'elleh = haa-'ulaa' either. This idea has probably already been put forward (or indeed knocked down) somewhere in the literature, for all I know, but there you go.
In either case, both definite articles would derive originally from unstressed demonstratives - a process so common it's barely worth commenting on. For example, all the Romance languages derive their definite articles from Latin demonstratives - usually illum/illa "that" (before the noun except in Romanian), but istum "that" in Sardinian. Likewise, the Coptic definite article pe- (m.)/te- (f.)/ne- (pl.) derives from the ancient Egyptian demonstrative pn (m.), tn (f.), nn (pl.) "this". Indeed, English "the" derives from the same old English word as "that". Come to think of it, I don't know of any definite articles offhand that don't derive from demonstratives; can you think of any?