Ad Hominem-Today, too many people use this as the same thing as a personal insult...this is indeed the most misunderstood and the most misapplied logical fallacy.
No no... here is what an ad hominem is: an error in reasoning, nothing more nothing less. It is basically like saying that your opponent is wrong not because there is something wrong with his reason, but just because he is an idiot.
Here is a more subtle and more common-place version of ad hominem logical fallacy.
John's argument about medicine is wrong because he got a 2.0 in his Medical school.
Next most commonly abused fallacy is the Straw Man fallacy...
I take for it to be common courtesy that when you arguing... you make your opponent's case as strong as possible first... and maybe later.. if you think he is wrong... you attack his argument then...
A straw man fallacy occurs when we misrepresent the opponent's case and purposefully make it weak just so we can have the pleasure of tearing him up..
Here is a good example of this...
I say for example, in order for you to be in time, you must have both mass and be subjected to light, as supported by Einstein's relativity. So if the mind survives the death of body, it will be outside of time.
And my opponent says, thats religious non-sense, mind surviving the death of body!?
Basically there... the claim that is getting attacked is one that I did not make, its a distortion of what I seem to have had in mind.
And finally, circular reasoning, the most common error on this site.
Circular reasoning is basically having your conclusion the same as your premises.
A typical example of this would be saying that you're right, but not supporting it with logical argument.
Here is an example of a valid argument..
All men are mortal
I am a man
I am mortal
Circular reasoning---conclusion comes ahead of the premises...
I am mortal...
Then my opponent asks me to justify this... and again... I say I am mortal and period...