An "unequalled" subject

Door Knob
Originally uploaded by Sandra Rocha

I saw this on a poster that was stuck on a classroom door. The subject referred to has been omitted because I want you to tell me the subject you think belongs there, and to tell me why you think it deserves that place:

[Insert subject here] is the unequalled agent of mental discipline and the embodiment of constructive and inventive thinking.

{It reads like a quote but no attribution was given on the poster. If anyone can give me a source, I'd be very grateful}

So over to you - give me a subject and give me your reasons.

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