Is there a good etymological reason why the potential form in Japanese requires the が particle?

Question

Is there a good etymological reason why the potential form in Japanese requires the が particle?

When dealing with the -たい suffix, which also requires the -が particle for what in other languages would be its object, I always conceptualised the word before -が acting as the subject of a "passive desiderative". This might actually historically be wrong (I am not sure), but it made it seem much more logical in my head.

So 私は寿司が食べたい then translates conceptually into "as for me, sushi is desired to be eaten", or changing the form into an adjective (which the -たい form kind of is): "as for me, sushi is desirable for eating". Thus "sushi" becomes the actual subject of the sentence, and the が particle becomes logical.

Now I am looking to see if it is possible to find a similar explanation for the reason why the が particle is used for the potential form… I can of course simply see the potential as a passive (quite logical, as the forms are identical for -いる/える verbs anyway), but that leaves me with a different meaning (e.g. "as for me, Japanese is written", rather than "I can write Japanese"). This would only make sense if the potential meaning directly derives from an earlier passive meaning, which I am not sure of.

Answer 1

I'll divide my answer in two parts.

Part 1: The connection with V-ことが出来ます and the tendency of using を

As I mentioned in the comment, I think there is a relationship between the が used in the potential form and the construction V-ことが出来ます.

Let's consider the example: 地図が読める / 地図を読むことができる.

Basically in Japanese we can distinguish these two cases:

1. [Subj]は[Obj]が[State predicate (状態述語)]
2. [Subj]が[Obj]を～することができる/～することができない

The state predicate is a predicate that expresses the object being in a certain status, and 1. in general indicates that the subject has the ability or desire of doing something.

In our example 読める/読みたい are state predicates (状態述語) and we can use が:

「私は地図が読める（expresses the condition of having the ability of）」

「私は地図が読みたい（expresses the condition of desiring)」

These are both correct.

In the second case, the sentence would be 私は地図を読むことができる. It happens often that the V-ことが出来ます part is changed into the potential form, which turns the sentence into 私は地図を読める. I think this is commonly used and accepted by native speakers as well, especially in spoken Japanese. However, I believe the most grammatically correct version would be with が.

A discussion around this subject can be also found at this link.

Second part: why が is used.

I think the discussion here pretty much answers your question. Let me just cut and paste the most relevant part:

Potential verbs are formed like 話【はな】す → 話【はな】せる, and so-called "passive verbs" and "spontaneous verbs" are formed via the same pattern. These other verb [types] are not formed quite as regularly as the potential, which has almost no exceptions, so these are listed as headwords as derived verbs in other dictionaries.

[Translator's note: ≒ as used below means "is roughly equivalent to".]

「謎【なぞ】を解【と】く "solve a riddle"」 →「謎【なぞ】が解【と】ける "a riddle can be solved"」≒「謎【なぞ】が解【と】かれる "a riddle is solved"」 (passive) 　

「気【き】を置【お】く "set down / aside one's attention / interest" (used idiomatically to mean "to be respectful of someone else's intent; to be relieved"」→「気【き】が置【お】ける "one's interest can be set down / side"」≒「気【き】が置【お】かれる "one's interest is set down / aside"」 (spontaneous)

The derivation process for potential verbs is the same as these. Consequently, the original tendency has been for potential verbs to not take the を object case.