@demonhunter,
I"ll bite...it seems odd to me that no one has replied to this already.
I like wiki's answer to the question:
http://en.wikipedia.org/wiki/Critical_thinking , but it seems like an honest, personal answer requires the specification of which assumptions one means to question. There seem to be several different versions of critical thinking available:
1) Conceptual/logical: one asks of an event "why, why, why". And one responds with the answer "If...then...therefore..."
2) Metaphysical/ontological: one asks of an event "why, why, why". And one responds with the answer of "How...then...that..."
3) Empirical/ scientific: one asks of an event "why, why, why". And one responds with the answer of "If...how...then...."
4) Historical: one asks of an event "why, why, why". And one responds with the answer of "If that...therefore then...now how..."
This answer is formulaic, i know, but i thought i might start the debate. i don't usually like answering homework type questions, but i liked your honesty in that regard. i like a good debate, but this is also the sort of question that might get the **** slinging...
i doubt that my response has overtly impacted your original idea about your eventual essay, but i enjoy the idea of possibly taking a part in someone's writing process, one way or the other. The above is nothing like a definition, but i'm curious as to the OP's definition: Happy writing!