I've seen this simple circuit design many times when the text is illustrating different points. Sometimes, again for illustrative purposes, the text may say that I3 is negative. When would this ever happen? If I3 were negative, the node between components 3 & 4 would have to be lower than ground, and, further, the red extraordinary node would have to be lower yet. It seems intuitive to me to conclude that the voltage at the node in red will never be at a lower potential than ground in this design. If I3 is negative, it would seem obvious to me that I2 must also be negative (with polarity defined as is it) - but when could this be the case? Perhaps I3 could be negative if different node is selected as the reference, but as it stands I cannot see a way that this claim would ever be true.