Basis
Grounds
noun, verbal noun
The root that this is based on is the basis and grounds for my argument. Look, it goes all the way down to the root problem!
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
彼が高所恐怖症だと思ったのには何か根拠があるんですか？
What basis is there for you to think that he has a fear of heights?