What does the verb 'describe' mean?

Question

In this following context, what does the verb 'describe' mean?

What is different between the verbs 'describe' and 'apply'?

Is the 'which' an object of 'describes'? I think the 'it' is dummy subject here.

Context:

The entire Pațțhãna, the last book of the Abhidhamma Pi†aka, which fills six bulky volumes, treats exclusively of these twenty-four conditions, or paccaya, which it first describes and then applies to all the innumerable mental and physical phenomena of existence.

Source: Fundamentals of Buddhism Four Lectures by Nyanatiloka Mahåthera

Answer 1

This seems to be a rather old, dated text, because of the use of the verb phrase "treat of" (meaning "cover" or "handle" a topic). That is not modern English. It smells like the 19th century.

The pronoun "it" refers back to the book ("it" first describes something...).

To describe a thing is to explain what it is like (an apple is red or green, and round, and tastes good). To apply a thing is to use it in the real world (I was taught about computers, and now I can apply that knowledge by writing some code).

So: the book (Patthana) first describes "these 24 conditions" and then applies (them) to the phenomena of existence. First the book explains what the conditions are like, and then it tells you how to use them in life (I suppose). Good luck :)