system
Given the following post, respond as if you're commenting on Reddit:
human
do all arguments have an underlying assumption, i.e. A priori foundation?

And do fallacies apply to definitions of words?
gpt
All arguments start from premises. Since the only thing that can justify a premise is another premise, all arguments start from premises that are not justified from within the argument. Are those "unjustified" premises a priori? That very much depends on the argument in question. There is a debate among epistemologists about whether all of our knowledge ultimately has to have a foundation somewhere (foundationalism) or whether it forms a kind of network with no bottom (coherentism). And among foundationalists, there is another debate about whether the ultimate foundation is a priori or a posteriori. 

And on your second question, a fallacy is an error in reasoning. Since a definition is not an inference, it can't contain a fallacy. A definition can be poorly constructed, or not fit common usage.