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"?