Why does voltage lead current by π/2 in a pure inductor or capacitor?

[thereddevils]

My book saya that in a pure circuit , the phase difference between the voltage and current must be pi/2 , otherwise it is not pure . Why it is so ? What is a pure circuit by the way ?

 

[Stonebridge]

It means a circuit with only capacitance or only inductance and no resistance.