Shikantaza is to practice or actualize emptiness. Although you can have a tentative understanding of it through your thinking, you should understand emptiness through your experience. You have an idea of emptiness and an idea of being, and you think that being and emptiness are opposites. But in Buddhism, both of these are ideas of being. The emptiness we mean is not like the idea you may have. You cannot reach a full understanding of emptiness with your thinking mind or with your feeling. That is why we practice zazen.
– Shunryu Suzuki
It radiates light and brightness;
When the wind comes
There arise ripples and waves.
There is a time for peaceful contemplation,
There is a time for dynamic action,
And all the time the lake remains itself.
~ Yongming Yanshou