form a worthwhile opinion for we need to have factual knowledge ___ our ...
This sentence is from an exercise book.The whole sentence is:
Without facts,we cannot form a worthwhile opinion for we need to have factual knowledge ___ our thinking.
The correct answer of the blank is:on which to base
The red colour part make me confused.
I hope somebody could be telling me the structure of the sentence.I can't fully understand such sentences.
请帮我理解从空处开始向後的这个部分