It’s from Mein Kampf:
“All
propaganda must be popular and its intellectual level must be adjusted
to the most limited intelligence among those it is addressed to.
Consequently the greater the mass it is intended to reach, the lower its
purely intellectual level will have to be ... The receptivity of the
great masses is very limited, their intelligence is small, but their
power of forgetting is enormous. In consequence of these facts, all
effective propaganda must be limited to a very few points and must harp
on these in slogans until the last member of the public understands what
you want him to understand by your slogan.”
