@firedragon
I will expand on this and provide the scriptural basis of my position. There are many, but two of them will do adequately. It comes from the Brihad-Aranyaka Upanisad which is considered the biggest and the oldest of the chief Upanisads of Hinduism.
Excerpts showing the neutral monistic Brahman is the foundation of external reality. The first comes from a conversation between Yagnavalka, the chief rishi/seer in the text and Gargi Vachnavi, an eminent female rishi of her times in the court of king Janaka.
The first section discusses what is the fundamental entity that is the basis of all existence.
The second section from the same chapter discussed what is the fundamental essence of all entities.
Eventually, the last segment of both conversation ends with the identical description for both thereby stating that they are the same.
In my opinion that is the key and unique (or at least the earliest as BHU was composed around 900 BCE based on linguist analysis of the type of Sanskrit being spoken here) insight of Hindu philosophy. The rest is expanding on the concept, clarifying it, justifying it and using cultural systems to develop practices that help one realize it.
BHU Chapter 3 section 8
Then (Gargi) Vacaknavi spoke. "Distinguished Brahmins!" she said. "I am go-
ing to ask this man two questions. If he can give me the answers to them, none
of you will be able to defeat him in a theological debate."
"Ask, Gargi"
2 She said: "I rise to challenge you, Yajnavalkya, with two questions, much as a
fierce warrior of Kasi or Videha, stringing his unstrung bow and taking two deadly
arrows in his hand, would rise to challenge a rival. Give me the answers to them!"
"Ask, Gargi."
3 She said: "The things above the sky, the things below the earth, and the things
between the earth and the sky, as well as all those things people here refer to as past,
present, and future—on what, Yajnavalkya, are all these woven back and forth?"
4 He replied: "The things above the sky, the things below the earth, and the
things between the earth and the sky, as well as all those things people here refer to
as past, present, and future—
on space (akasa), Gargi, are all these woven back and forth."
5 She responded: "All honor to you, Yajnavalkya. You really cleared that up for
me! Get ready for the second."
"Ask, Gargi."
"
On what, then, is space woven back and forth?"
8 He replied: "T
hat, Gargi, is the imperishable (Brahman), and Brahmins refer to it like
this—it is neither coarse nor fine; it is neither short nor long; it has neither blood
nor fat; it is without shadow or darkness; it is without air or space; it is without
contact; it has no taste or smell; it is without sight or hearing; it is without speech or
mind; it is without energy, breath, or mouth; it is beyond measure; it has nothing
within it or outside of it; it does not eat anything; and no one eats it.
9 "This is the imperishable, Gargi, at whose command the sun and the moon
stand apart. This is the imperishable, Gargi, at whose command the earth and the
sky stand apart. This is the imperishable, Gargi, at whose command seconds and
hours, days and nights, fortnights and months, seasons and years stand apart. This is
the imperishable, Gargi, at whose command rivers flow from the snowy mountains
in their respective directions, some to the east and others to the west. This is the
imperishable, Gargi, at whose command people flatter donors, and gods are de-
pendent on patrons of sacrifices, and forefathers on ancestral offerings.
10 "Without knowing this imperishable, Gargi, even if a man were to make of-
ferings, to offer sacrifices, and to perform austerities in this world for many
thousands of years, all that would come to naught. Pitiful is the man, Gargi, who
departs from this world without knowing this imperishable. But a man who departs
from this world after he has come to know this imperishable—he, Gargi, is a Brah-
min.
11 "
This is the imperishable, Gargi, which sees but can't be seen; which hears
but can't be heard; which thinks but can't be thought of; which perceives but can't
be perceived. Besides this imperishable, there is no one that sees, no one that hears,
no one that thinks, and no one that perceives.
"On this very imperishable, Gargi, space is woven back and forth."
12 "Distinguished Brahmins!" said Gargi. "You should consider yourself lucky
if you escape from this man by merely paying him your respects. None of you will
ever defeat him in a theological debate."
BHU Chapter 3 Section 7
Yajnavalkya, Now tell us
who the inner controller is."
3 "This
self (atman) of yours who is present within but is different from the
earth, whom the earth does not know, whose body is the earth, and who controls the
earth from within—he is the inner controller, the immortal.
4 "This self of yours who is present within but is different from the waters,
whom the waters do not know, whose body is the waters, and who controls the wa-
ters from within—he is the inner controller, the immortal.
5 "This self of yours who is present within but is different from the fire, whom
the fire does not know, whose body is the fire, and who controls the fire from
within—he is the inner controller, the immortal.
6 "This self of yours who is present within but is different from the intermediate
region, whom the intermediate region does not know, whose body is the intermedi-
ate region, and who controls the intermediate region from within—he is the inner
controller, the immortal.
7 "This self of yours who is present within but is different from the wind, whom
the wind does not know, whose body is the wind, and who controls the wind from
within—he is the inner controller, the immortal.
8 "This self of yours who is present within but is different from the sky, whom
the sky does not know, whose body is the sky, and who controls the sky from
within—he is the inner controller, the immortal.
9 "This self of yours who is present within but is different from the sun, whom
the sun does not know, whose body is the sun, and who controls the sun from
within—he is the inner controller, the immortal.
10 "This self of yours who is present within but is different from the quarters,
whom the quarters do not know, whose body is the quarters, and who controls the
quarters from within—he is the inner controller, the immortal.
11 "This self of yours who is present within but is different from the moon and
the stars, whom the moon and the stars do not know, whose body is the moon and
the stars, and who controls the moon and the stars from within—he is the inner
controller, the immortal.
12 "This self of yours who is present within but is different from space, whom
space does not know, whose body is space, and who controls space from within—he
is the inner controller, the immortal.
13 "This self of yours who is present within but is different from darkness,
whom darkness does not know, whose body is darkness, and who controls darkness
from within—he is the inner controller, the immortal.
14 "This self of yours who is present within but is different from light, whom
light does not know, whose body is light, and who controls light from within—he is
the inner controller, the immortal."
That was with respect to the divine sphere. 15 What follows is with respect to
beings.
"This self of yours who is present within but is different from all beings, whom
all beings do not know, whose body is all beings, and who controls all beings from
within—he is the inner controller, the immortal."
That was with respect to beings.16 What follows is with respect to the body
(atman).
"This self of yours who is present within but is different from the breath, whom
the breath does not know, whose body is the breath, and who controls the breath
from within—he is the inner controller, the immortal.
17 "This self of yours who is present within but is different from speech, whom
speech does not know, whose body is speech, and who controls speech from
within—he is the inner controller, the immortal.
18 "This self of yours who is present within but is different from sight, whom
sight does not know, whose body is sight, and who controls sight from within—he
is the inner controller, the immortal.
19 "This self of yours who is present within but is different from hearing, whom
hearing does not know, whose body is hearing, and who controls hearing from
within—he is the inner controller, the immortal.
20 "This self of yours who is present within but is different from the mind,
whom the mind does not know, whose body is the mind, and who controls the mind
from within—he is the inner controller, the immortal.
21 "This self of yours who is present within but is different from the skin, whom
the skin does not know, whose body is the skin, and who controls the skin from
within—he is the inner controller, the immortal.
22 "This self of yours who is present within but is different from perception,
whom perception does not know, whose body is perception, and who controls per-
ception from within—he is the inner controller, the immortal.
23 "This self of yours who is present within but is different from the semen,
whom the semen does not know, whose body is the semen, and who controls the
semen from within—he is the inner controller, the immortal.
"
He sees, but he can't be seen; he hears, but he can't be heard; he thinks, but he
can't be thought of; he perceives, but he can't be perceived. Besides him, there is no
one who sees, no one who hears, no one who thinks, and no one who perceives. It is
this self of yours who is the inner controller, the immortal. All besides this is grief."
Thereupon, Uddalaka Aruni fell silent.
I am using a translation from a printed book, but a good translation can be had here
Yajnavalkya and Gargi (II) [Section VIII]
Yajnavalkya and Uddalaka [Section VII]
