a common tautology

     A "tautology" is a classic error of logic--one on the list students were once taught to recognize and avoid. Someone using a tautology defines the thing or idea in question by defining the thing or idea in question--"it is what it is" is a tautology. A definition can make a sound  basis for an argument, but it is not a substitute for an explanation. A common tautology, or set of tautologies, that you can hear on TV crime shows sounds like this--
Question--"why did the suspect do these crazy things?"
Answer--"because he's crazy."--or "because he's a psychopath."
The "why" part of the question has not been answered.
Illogical.

The "argument by definition", in the case of the TV psychopath, would not be particularly useful, but would run like this--
A psychopath is defined as someone who commits violent crime for no discernible motive. He has no motive of profit, and no motive of protecting his reputation, two of the most common criminal motives. This criminal commits violent crime without any discernible motive, therefore this criminal is a psychopath.
 Logical, but not particularly useful.

yes, they could all be wrong

     Don't fall into this trap--two people are arguing about something, and they expect you to take sides--to decide who is right and who is wrong. Assuming that one of them has the "right" answer, you will be expected to help that person win the day. But what if they are both wrong? For example, suppose that one is maintaining that 1+1=3, and one is maintaining with equal emotion that 1+1=5?  Which of them is right? Will you "side" with one of them? Or will you tell them that they are both wrong, and start a third party? 
     The two wrong people may believe that they are standing on a firm logical foundation, since two mutually exclusive propositions cannot both be true. Mutually exclusive would mean that if one is true, the other must be false--as in it's day or night outside--it is obvious, without formal logic, that only one of these can be true at any given time, and in any given place ( the clever will bring up time zones here, in an attempt to prove that these could both be true--not so).   In the example of 1+1, however, the propositions are not mutually exclusive--if 1+1 does not equal 3 ( and it doesn't ), it does not necessarily follow that 1+1=5.