Because most people when they begin to argue about philosophy tend to start in media res and without any epistemic foundation what usually happens is that both sides become aggravated by what seems to be a simple inability to understand the other’s arguments. In reality what has occurred is that both sides have fundamentally different assumptions which in turn imply a distinct approach to how both reality and knowledge is known. Let us take mathematics for example (so as to shy away from any preconceived bias towards any given philosophic assumptions). If you were to pick up a book on Euclidean geometry and attempt to construct or prove even the most simplest proofs without any basic assumptions you would inevitably fail. This is because the basis of geometry lies on the principle assumption of the definition of a line as well their ability to be parallel. Conversely before we can do even basic arithmetic we must first assume the function and process of addition and subtraction as well as the definition of number else we become a slave to an unknown process. Just so in philosophy, if the basic definitions, either in terminology or primal law, are not consistently assumed then there can be no grounds from which we can argue.
The intuitive student may then ask how we are to escape a circle where the most difficult thing to demonstrate (indeed often impossible) must be assumed before the more simple things can be explained, for it would seem that this process (so long as we are honest in it) leads to no other alternative but a sort of skepticism. It is true that there must be a commonality of assumptions before we can move to the next progressive level of philosophic thought, but doing so does not in any way neglect the knowledge we have. We must remember that while our philosophic model of the world must conform to a sort of logical process, the reality which it attempts to render is bound by no such limitation. In fact, it is because we cannot reduce some known truth about the way the world is to some logical provision that it is indeed an assumed proposition in the first place. We do not need to logically prove Aristotle’s law of non-contradiction (the law of non-contradiction says that something cannot be and not be at the same time and in the same respect) to confidently utilize it within a philosophic model precisely because it is that very thing which allows logic to operate as well as something which conforms as best we can perceive to the world.
We must make these basic assumptions which we intuit not through the logical agreement but because they become the basic foundation from which we can make logical sense of the world. If one where to disagree and claim that Aristotle was confused and this were no law, and we understanding it to be true thought to argue against them, there could be no means to argue, no foundation by which we would understand each other. Just as the rhetorical process of argumentation becomes futile when those who speak do so without a common language, so philosophy becomes futile when there are no commonly accepted principles.