How should satipatthana be understood in regard to external/internal minds? "In this way he remains focused internally on the mind in &amp; of itself, or externally on the mind in &amp; of itself, or both internally &amp; externally on the mind in &amp; of itself. (Thanissaro)


How should the fragment above be understood and practiced in detail, in particular, considering the last two alternatives: externally and internally/externally?
Here is a short quote from an article on BuddhaNet called &quot;The Vipassana Retreat - The Six Sense-spheres&quot;. In here it's talked about how internally and externally should be understood.

The Vipassana Retreat

The Six Sense-spheres

The teaching of the sense-spheres is to be found in the Contemplation of Mind (dhammanupassana) in the text we are following: the Satipatthana Sutta, which by now is familiar to you as the Four Establishments of Mindfulness.
After each section in the text in the Satipathana Sutta, you will find this passage: &quot;In this way he abides contemplating the body as a body (or feelings, mind states and mental phenomena) internally, externally, and both internally and externally&quot;. What does this mean? It means that the focus of one's attention changes from the subjective (internal) to the objective (external) and by &quot;both&quot; is meant the understanding of the interrelationship or interdependence.
The focus of the practice so far has been mainly introspective; now the watchfulness or attentiveness can be expanded to include the external as well. That is, the attention is switched from the subjective to the objective. This is done by orientating to the sense-spheres, which are about the relationship between oneself and the outer world. Practising both internal and external satipatthanas can prevent self-absorption, and achieve a skilled balanced between introversion and extroversion.
The importance of contemplation of the sense-spheres is that it directs awareness to the six “internal” and “external” sense-spheres and the fetters (samyojana) arising in dependence on them. Although a fetter arises dependent on sense and object, the attaching nature of such a fetter should not be attributed to the senses or objects themselves, but to the influence of the hankering pull of desire (tanha)...

The article says further on how to make an orientation to a sense-door:

Orientation to a Sense-door
To make an orientation to a sense-door, you start by literally coming to your senses - seeing, hearing, tasting, touching and smelling. These are the five sense-doors or sense bases; the 'sixth sense' is 'consciousness of something', which is the mind-base with its eye-consciousness, ear-consciousness, etc. You also need to be aware of the senses internally as well as externally. That is, the organs and their sense objects: nose/smell, tongue/taste, body/tactile objects, ear/sound, mind/mind-objects or consciousness.
Attentiveness or 'presence of mind' at one of the sense-doors during a sense impression is the way to practice. For example, most people are predominantly visual, so being attentive at the eye-door allows you to notice the effects of the contact between the eye and the visible objects and how you are relating to them.
The process is this: there is the eye (the internal base), and a visible object (the external base). With contact or a sense impression between the sense-door and external object, consciousness arises followed by feeling. The moment of consciousness ordinarily is too rapid to catch while the feeling tone can be more easily known and apprehended.
This orientation to a sense-door brings awareness of what is happening during the moment of contact or the sense impression, and with it the ability to monitor the associated feelings and consciousness that arises. When this feeling tone is apprehended, the link to liking and disliking is broken and therefore one is free at that moment from conditioned suffering.

Here is a quote from the Wiki on the 6 sense bases:

Ṣaḍāyatana (Sanskrit) or saḷāyatana (Pāli) means the six sense bases (Pāli, Skt.: āyatana), that is, the sense organs and their objects. These are:
Eye and Vision
Ear and Hearing
Nose and Olfaction
Tongue and Taste
Skin and Touch
Mind and Thought
That is, in Buddhism, the sixth &quot;internal&quot; and &quot;external&quot; sense bases are: mind (Skt., manas; Pali, mano); and, thought (along with memory and emotion) (Skt., dharma; Pali, dhamma).
Ṣaḍāyatana is the fifth link in the Twelve Nidānas of Pratitya-Samutpada (Dependent Origination) and thus likewise in the fifth position on the Bhavacakra (Wheel of Becoming). Ṣaḍāyatana (Sense Gates) is dependent on Nāmarūpa (Name and Form) as condition before it can exist.