How does one interpret the Brahma Sutras, when the Sutras by themselves are vague?

Question

Because the Brahma Sutras are almost impossible to understand without the appropriate context, and because very few people read them without the help of a commentary (like that of Ramanuja's or Shankara's), how did scholars know the meaning of each Sutra when the Sutras by themselves are vague?

For example, sutra 1.1.5, or the 5th sutra, in Sanskrit is eekshaternaashabdam, which translated literally is:

On account of thinking, that which is not based on the Scriptures, is not.

Ramanuja takes this to mean:

On account of thinking, that which is not based on the scriptures [viz the Pradhana] is not [the Sat mentioned in the scriptural text relating to the first cause].

But how come Badarayana left out important words like Pradhana, Sat, and First Cause from this Sutra, and left it to later Acharyas to fill in the blanks?

This is why I asked in my previous question if Badarayana himself ever wrote a commentary on the Brahma Sutras he compiled, but it turns out he never did.

So how does one know what each Sutra is really talking about? In the 5th Sutra above particularly, how does one know that "is not", is referring to "is not the first cause"?

Answer 1

Before going to actual answer let's understand what is mean by a "Sutra" from Hinduism point of view.

A "Sutra" is like a theorem distilled into few words or syllables, around which teachings of ritual, philosophy, grammar, or any field of knowledge can be woven. Like a short rule. Thus Sutra briefly states the message while a Shloka is a verse that conveys the complete message.

We come to know from the index that the subject of sutras 1-11 from Adhyaya 1 Paad 1 is to show that Brahman is the ultimate cause of this universe and not the Jad Prakriti (Nature).

Now let's look at your doubt. "

But how come Badarayana left out important words like Pradhana, Sat, and First Cause from this Sutra"
and "does one know what each Sutra is really talking about? How does one know that "is not", is referring to "is not the first cause"

ईक्षतेर्न, अशब्दम् ॥ ५ ॥

īkṣaterna aśabdam || 5 ||

Whenever the topic of creation is discussed in Upanishads the word "eksha" which is said to be Chetan (alive), is used. But Jad-Prakriti is not alive . Brahman 'saw/deliberated/desired' and therefore we conclude that It is chetana vastu. So the Prakriti is not the primary cause of this material creation , and also this is without Shruti-Pramana.

So from looking at the word "aśabdam" ,seeing its meaning ,how it's used in other Shruti's the scholars came to know about the meaning. Also the format of sutras is short. The word "Sat and Pradhana" are automatically involved in the sutra (because Brahman is "SAT"). So Badrayana not left them ,as sutras are short and required other shruti pramanas to interpret.

The discussion mainly refers to the sixth chapter of the Chhândogya Upanishad. Also see Aitareya 1.1.1.