I have lately found myself using two particular strategies quite often during discussions and want to make sure that their logical structure is valid. So, I thought what better place to have them dismantled than LW :)
 The first strategy involves sending a hypothetical example's equivalent back in time and using the present knowledge of the outcome as a justification for the validity or not of the argument. The last time I used this was when someone tried to convince me that IQ is the main factor for human value by asking me which one is superior, a technologically developed, high IQ culture vs an under-developed mid IQ one?
I responded that I can not rationaly know what to do based on only this information. When pushed on why and on making a choice, I responded that if you were asking me this question about pre-war Germany in the place of the highly developed country using your own logic you would choose Germany as superior but we now know that the 'superior' country was morally inferior (I assumed correctly that they accept similar definitions of good and evil and German actions in the war were evil). With the benefit of hindsight we now know that this would be the wrong decision so their argument is demonstrably wrong.
Now, I don't want to get into this argument here. I just want to know if the strategy I used is logicaly valid as they did not accept it and instead, more or less, accused me of sophistry.
 The second strategy is more suspect to my estimation but I am not sure why. In this method I demonstrate humanities miniscule understanding of reality (when put in proper perspective) and use this as a basis for a kind of attitude. Here is an example:
When discussing whether life has meaning or not one answer I use is a pragmatic one. The issue at hand is deciding how to act. In other words which belief to use as a motivation for action. There are two epistemic possibilites:
- [2.1] life has meaning
- [2.2] life does not have meaning.
First of all, we do not know if life has meaning or can estimate with any reasonable confidence. We can estimate based on current data but our data is tiny compared to the whole of reality. Therefore, we should always act as if [2.1] is true on the basis that, if true, we (personally or humanity as a whole) might understand and even contribute towards it. If [2.2] is true on the other hand the things to be lost (like effort, comfort etc.) are nothing in comparison.
(Woops, I just casualy introduced a discussion starter about the meaning of life... - sorry about that :P - Feel free to respond on whether the presented argument is sound but please do it in a seperate comment from the one discussing whether it is logically valid)