Although I agree in general terms, I offer this passage as an example of using internal/external as an approach to seeing the not-self nature of phenomena:Greetings, I'm enjoying the article a great deal but one important question I have is, who is it that's proposing this idea "that we are all One?" What groups or individuals are you referring to? Do they have specific doctrines, beliefs and practices around this idea of Oneness? And are you representing them accurately? Being specific in this way seems important because right now your presentation of Oneness seems a bit cliche or stereotypical. In other words, if you are not more specific about who holds this view of Oneness and if you don't articulate their position at least somewhat accurately, then your presentation of Oneness seems more like a caricature, a straw-man, that you can then punch down.
And what is the earth element? The earth element may be interior or exterior. And what is the interior earth element? Anything hard, solid, and organic that’s internal, pertaining to an individual. This includes head hair, body hair, nails, teeth, skin, flesh, sinews, bones, bone marrow, kidneys, heart, liver, diaphragm, spleen, lungs, intestines, mesentery, undigested food, feces, or anything else hard, solid, and organic that’s internal, pertaining to an individual. This is called the interior earth element. The interior earth element and the exterior earth element are just the earth element. This should be truly seen with proper understanding like this: ‘This is not mine, I am not this, this is not my self.’ When you really see with proper understanding, you reject the earth element, detaching the mind from the earth element.