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All answers having "Reason" as clue:

3 Character Answers:

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AGE

WHY



4 Character Answers:

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NOUS

PLEA

SAKE



5 Character Answers:

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AGEOF

ARGUE

BASIS

CAUSE

INFER

LOGIC

SENSE

THINK



6 Character Answers:

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DEDUCE

MOTIVE

SANITY

SENSES

WITHIN



7 Character Answers:

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PURPOSE



13 Character Answers:

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JUSTIFICATION



15 Character Answers:

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LOGICALTHINKING