The Makedonian phalanx -- why such depth?

[Paullus Scipio]

Paralus wrote:

The most important activity of an army is fighting a battle and, hopefully, winning. If your statement is correct then the most important officer at "platoon level" is the half file leader. Seems Alexander's army did not have such - as far as I can recall. We have file leaders (and closers) but no attested haf file colleagues.

...I'm afraid your recollection is incorrect, but then no-one can be expected to remember every piece of minutiae about Alexander or his army - not even such a prominent member of the "Pothos" forum as yourself !
In fact we do hear of dekadarchs(file leaders), dimoiritai(double pay men) and two dekastateroi ( ten-stater men) (Arrian Anabasis 7.23) where the incorporation of Persians into the Phalanx is being described. These are generally agreed as 'file leader', 'half-file leader' and two 'half-file closers' - which points to actual fighting taking place in half-files 8 deep, since why would you place a man with a pike at rank nine, where he is completely useless?...
Alternately, Arrian may not be correct, and the two 'ten stater men' are not half file closers at all and do not stand at the rear, since once more, what use is a man with a pike at the back? ( in the manuals these rear rank men are called ' ouragoi lit: followers/covering men/bringers up of the rear)' Thus they could be 'quarter file leaders', which would facilitate forming synaspismos (locked shields, 4 deep), with a front rank bristling with pikes backed up by archers. Lest it be argued that this is a peculiarity of A.'s re-organisation, clearly the terms existed beforehand in order for them to be applied. Further, support for this comes from a fragment of Papyri ( Pros. Ptol. 2273-2287) from Ptolemy 1's reign which refers to dekanikoi (file leaders) and dimoiritai ( double pay men/half file leaders) and Ptolemy's Phalanx was certainly a traditional Macedonian one !
Needless to say, the Hellenistic manuals also refer to 'file leaders' and 'half-file leaders' - Aelian even claims he is referring to Alexander's Phalanx. Aelian also says:
"It is necessary that the 'File leaders' should exceed the others in bravery; and next to them the 'Half-file leaders' and also the 'File closers' ( ouragoi)."
Why should the 'Half File Leaders' need to be the second bravest, if they skulked eight ranks back in action? Indeed why have 'Half file Leaders' at all? This clearly implies that the 'Half File Leaders' actually fought in the front rank. Since any 'rout' must necessarily begin from the rear, it is clear that the ouragoi needed to be brave men who would stand their ground and prevent runaways. But notice that an 'ouragos' brings up the rear of each 'half-file' - which if he is to be at the rear, and not in the eighth rank half way back, also means that the Phalanx fights in half -files, with the ouragoi at 'eighth' and 'sixteenth' in the file thus forming the rear rank !

You insistence on the one line in Xenophon's Anabasis that the fighting depths of Greek pahalanxes was four is a stretch of one single line. I might as well state that Thucydides' mention of Spartan "felt caps" at Sphacteria means Spartan hoplites always wore same, as did the other Greeks. One mercenary army; one place and one pre-dinner display drill. Xenophon himself later states that the army fell into battle order (in another drill at Byzantium if I remeber) eight deep. You hang much wedding tackle on a single line...

You'll have to be more specific about your Byzantium reference - the city is referred to too many times in Xenophon for me to track down what you are referring to.....
I'm afraid that is an old 'canard' that the Spartans wore 'felt caps' at Sphacteria. In fact some/many of them wore bronze helmets in the conical shape of the 'pilos' cap...several forms of Greek headgear - the 'pilos cap', 'Boeotian cap', and the 'petasos hat', were all made in bronze as helmets.
As to hanging one's 'wedding tackle', I will bet on Xenophon - a General of Hoplites, let it be remembered- than on no information at all as to how the Phalanx actually fought ! And I will bet on Kallisthenes, Alexander's official historian for Macedonian depth of eight as 'fighting depth', again rather than no information at all. Notice that Polybius, who also knew a thing or two about Phalanxes, does not comment on the 'eight' depth as being in any way out of the ordinary, which implies it was the norm.
The point is that there is plenty of evidence for the Phalanx, be it Greek or Macedonian, actually fighting in 'Half files' in close order, rather than 'Files' which we know "normally" stood in 'Open ' order.