True Story: As Freshmen in Architectural school, we had practicing NY Architects come in weekly to critique our work. These brilliant, talented and egocentric people weren't professors teaching us how to design; they just commented on work in progress - what we had already designed.
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Of course, we all ran after class to look this up, and there were lots of discussions over the next week. A justification is a solid reason in advance to do something. It propels what you do in the right direction. Even after you've taken action, the justification holds up. A rationalization is an explanation after the fact that usually amounts to an excuse. You are coming up with reasons that don't justify the action you took, but seek to explain it away. The two words often get mixed up, and are considered synonyms, but if you get the meaning behind this piece of teaching, that is what is important.