～は～の一つだ【は～のひとつだ】

The structure ～は～の一つだ is used to express that something is one of a group or category. It identifies a particular item, person, or concept as belonging to a larger set.

The basic pattern is:

Noun A + は + Noun B + の一つだ

where Noun A is the specific item and Noun B is the group or category it belongs to.

Polite and casual forms

In casual speech, the sentence ends with ひとつ一つだ. In polite speech, use ひとつ一つです.

Using 中 for emphasis

You can add のなか中 after Noun B to emphasize "among" the group:

Related expressions

When the group members are people, you may see ひとり一人だ (ひとりだ) instead of ひとつ一つだ. For animals or other counted objects, the appropriate counter may replace ひとつ一つ.

ひとつ一つだ is still common even for abstract concepts, reasons, or causes.