The six senses and the five aggregates are all external according to Buddhism, but we need to look at what ‘external’ really means in this context. A more nuanced view of external means lacking in, or totally devoid of inner nature or spirit.
This is to say that our Buddha-nature or atman is not an external thing which even includes our feelings and thoughts. Our Buddha-nature or atman is first-person but also primordially internal. And anyone who would dismiss the Buddha-nature because it is not apparent to the six senses or because it is unthinkable, fails to understand basic Buddhism.
Buddhism is really talking about the true self versus the external illusory self—a construct of a “me” which is the illusory self, kind of like the voice in our head. All this is external. It is hardly internal where lies the holy grail of our quest.
The Zennist is on a quest for this grail. But as the reader can see the difference between external and internal is quite subtle and difficult to comprehend.
All the koan cases we love to read but find almost impossible to understand have the sweet fragrance of truth. But our approach to these koans is the same as our approach to Zen overall it is too external.
Thus far, all of our endeavors to find the truth have been pretty much external. Philosophy seems perhaps more directed towards the internal or the first-person. But this is no pat on the back for philosophy. It too is adrift on this vast external ocean that lies before us.