One object

There is only one way to order it, so that P1= 1

Two objects

It is not as trivial as one object, but it's still very simple.

1 2 or 2 1

There are 2 ways to order it.

Three objects

Then things start to get more interesting.

1 2 3 ,  1 3 2

2 1 3,   2 3 1

3 1 2,   3 2  1

There are 6 ways to order it out, and you can see we separate them in three subgroups (three colomns),

it is not necessary, but it has the advantage to do this way, make you have a clear idea how they are arranged.