Toward the end of his life, Suzuki Roshi was asked to present his teachings in a nutshell. He apparently answered impermanence.
Realizing impermanence does seem sufficient for realizing selflessness. All and any content of awareness comes and goes, including everything that has to do with this human self. And what is left is the Ground – timelessness, awareness, emptiness – which all the shifting content arises within and as.
That which is always changing reveals that which does not change. The timeless present that time and change unfolds within and as.
It seems that impermanence and selflessness is already and always in awareness. Yet, this is temporarily clouded over by attempts to believe in ideas – which makes it appear as if something is permanent and that there is a separate and fixed I somewhere, usually placed on this human self or even pure awareness. And these attempted beliefs temporarily prevents the noticing of what is already alive in awareness: impermanence and selflessness.
The mind is aware of its own nature, yet tries to believe in ideas (which are always incomplete and ultimately false), and experiences stress through this dissonance. All that is needed is for the mind to be aware of what it is already aware of. To bring this already existing noticing of impermanence and selflessness into conscious awareness. To consciously become familiar with it and trust it.