RTP: $\displaystyle E[X]=\sum_{i=1}^{\infty} P[X\geq{i}]$

If I expand this algebraically, I can see it

$\displaystyle \sum_{i=1}^{\infty} P[X\geq{i}]= (P[X=1]+P[X=2]+...)+(P[X=2]+P[X=3]+...)+....$

$\displaystyle =P[X=1]+2P[X=2]+.....$

But I'm having trouble understanding the proof as given by the text and wikipedia, in particular the interchange of the order of summation

Given proof

$\displaystyle \sum_{i=1}^{\infty} P[X\geq{i}]= \sum_{i=1}^{\infty}\sum_{j=i}^{\infty} P[X=j]$

Then we interchange the order of summation

$\displaystyle \sum_{i=1}^{\infty} P[X\geq{i}]= \sum_{j=1}^{\infty}\sum_{i=1}^{j} P[X=j]$

This step, I do not understand. Given that $\displaystyle i$ does not appear in the summation of $\displaystyle \sum_{i=1}^{j} P[X=j]$, how am I supposed to expand this?