Cantor was behind this fixed idea (3,6)

I believe the answer is:

set theory

'cantor was behind this' is the definition.
(mathematician Georg Cantor devised set theory)

'fixed idea' is the wordplay.
'fixed' becomes 'set' (synonyms).
'idea' becomes 'theory' (theory is a kind of idea).
'set'+'theory'='SET THEORY'

(Another definition for set theory that I've seen is " Cantor started this".)

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