2/Historical Note

Historical Note on 2|$2$ (two)
The number 2|$2$ (two) is understood to have been treated as a special case of a number from the earliest historical records.

Many early languages has specific forms of nouns for when two of an object are under consideration, as well as different forms for singular and plural.

The ancient Greeks, in addition to having problems with the idea that $1$ is a number, also had trouble defining $2$ as such:
 * While it has a beginning and an end, it has no middle
 * Multiplication by $2$ consists merely of adding a number to itself, and multiplication was expected to do more than just add.

Thus $2$ was an exceptional case.

To the Pythagoreans, whether or not 2|two ($2$) was actually a number or not, it was considered to be the first female number, and said to personify the principle of diversity.

In contrast, the odd numbers were considered to be male.

The dyad, as such, is never specifically defined in, but introduced without definition in Power of Two is Even-Times Even Only.