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.
One of my classmates was enduring a particular brutal critique ("Why did you do this? Why is this here?") as the Architect slammed the pinned up blueprint with the back of his hand. My classmate tried to explain. Then, this frightening genius of a man gave us all one of my life's most valuable lessons. He stopped his criticism, and said, "I want you all to go to a library tonight and look up in a big dictionary the definitions of justification and rationalization. Never rationalize what you create."
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.