Why does 目 have a counting sense to it?


Why does 目 get used in situations like 二個目? How did it come about getting the
meaning of eye as well as some counting meaning?




I think it's related to the sense mentioned by Brandon, from an underlying
idea of "something that sticks out visually", hence the use of 目{め} "eye".
This same sense of "visually notable or prominent feature" gives rise to many
other uses of 目{め}, such as 結{むす}び目{め} "knot" from 結{むす}び "tying" + 目{め}
"visually prominent feature", or 籠{かご}目{め} "lattice" from 籠{かご} "basket,
basket-style weave" + 目{め} "visually prominent feature". It also attaches to
adjectives (though it is generally spelled in kana then), as in 長{なが}め
"longish, on the long side".

FWIW, Shogakukan's entry for 目{め} breaks the definitions down a bit
differently than Daijisen. Some of the definitions that are relevant to ideas
of counting include (my additions in [square brackets]):

> (3) 見る対象をいう。  
>  Indicates the object of seeing/looking.

or:

> (5) 連続する、物と物との隙間（すきま）。間の区切り。区切りをつける線条。また、そのように刻まれたもの。  
>  Gap between successive items. Separation between. Line separating things.
> Or, a notch cut as such a line.

or:

> (6)
> 空間的、時間的な切れ目。二つの物、あるいは二つの事態の区切りや接点。転じて、物の条理、また計量の区切りや、単位をいう。多くの場合、動詞の連用形と複合して用いられる。「切れ目」「切り目」「分け目」「折り目」「裂け目」「境目」「合わせ目」「繋ぎ目」「綴じ目」「くけ目」など。  
>  A spatial or temporal gap / break. [Note that this definition uses the word
> being defined...] The divider or point of contact between two things or
> situations. By extension, a division or unit in the reason for or
> measurement of things. In most cases, used in combination with the
> _ren'yōkei_ [continuative form, i.e. _-masu_ stem] of a verb. _Kireme_
> "break/gap", _kirime_ "cut", _wakeme_ "split", _orime_ "bend, fold",
> _sakeme_ "rip, tear", _sakaime_ "boundary, border line", _awaseme_ "join,
> seam", _tsunagime_ "connection, joint", _tojime_ "binding", _kukeme_
> "stitch", etc.

or:

> Ⅲ 〔接尾〕 (Suffix)  
>  1 数詞のあとに付いて、初めから数え進んでひと区切りをつけた、その区切りまでの数を表すのに用いる。「二番目」  
>  Attached after a counter [such as 個{こ} or 番{ばん}], used to express the
> number counted when counting the divisions between items from the start
> [that is, the ordinal or positional number]. _Nibanme_ "second"  
>  .  
>  2
> 形容詞の語幹、動詞の連用形などに付いて、そのような度合、加減、性質、傾向の意味を添える。「細め」「長め」「控えめ」「おさえめ」など。現代では、形容詞の連体形に付けていう場合もみられる。「細いめ」「長いめ」など。  
>  Attached to forms such as an adjective's stem or the _ren'yōkei_ of a verb,
> adds a sense of that kind of degree, extent, quality, or tendency. _Hosome_
> "slender-ish", _nagame_ "long-ish", _hikaeme_ "moderate, with reserve",
> _osaeme_ "restrained", etc. In modern Japanese, also seen attached to the
> _rentaikei_ [attributive form, same as the dictionary form] of adjectives.
> _Hosoi-me_ "slender-ish", _nagai-me_ "long-ish", etc.

That's a lot to digest, so:

### TL;DR:

The word 目{め} is generally translated just as "eye", but idiomatically, it is
used to express a wide array of ideas related to the basic sense, " **visually
prominent feature** ". This is one of the more productive and useful words in
Japanese, and learning the many uses of this term will serve you well.

