Consider the following circuit,
Now consider this,
How come with the RC circuit, we have a constant -90 phase shift. I don't understand how the "voltage lags current by 90 degrees" comes into play here. Surely, we're just plotting the transfer function right (i.e Vout/Vin) so who cares about current? And even if a capacitor provides a constant -90 degree shift, why doesn't it happen with just the capacitor circuit?