A 6th tool is anything that is not part of the 5 tools. We have narrowed skill sets down into 5 easy to use, easy to reference buckets. But there is more out there. It may be very difficult to scout and/or predict, but I know there are players who either have exceptional grit and ability to grind out success, or who is very good at all 5 tools but not great at any one... alas the "6th tool."
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Amazon: Going with the Pitch SE
Author of "Going with the Pitch: Adjusting to Baseball, School and Life as a Division I College Athlete" (Second Edition)