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Adjective: simple-minded  'sim-pul'mIn-did
  1. Lacking subtlety and insight
    "a simple-minded argument";
    - simplistic
  2. Lacking mental capacity and subtlety
    "I was simple-minded enough to fall for one man's lies";
    - dim-witted [informal], simple

See also: mentally retarded, naif, naïf, naive, naïve, retarded