"The starting point or principle is more than half the whole matter." - Aristole, Nicomachaean Ethics
When Thomas Jefferson wrote in the famous American Declaration of Independence, “We hold these truths to be self-evident” – (Jefferson had originally written “sacred and undeniable”) – “that all men are created equal, that they are endowed by their creator with certain unalienable rights,” and so on, he was laying down the basis upon which the claims of the document would rest. He was enough of a philosopher to know that he had to begin with clear assumed principles; otherwise, he would be making arguments in mid-air. Such assumed first principles are a necessary aspect of any given line of thought. They cannot be proven. They are either considered undeniable (hence the term “self-evident”) like the principle of non-contradiction, or they are perceived intuitively like the claim that moral good and evil exist, or they are held on what are thought to be reasonable grounds such as the dogmas of a revealed religion. These foundational principles are not argued for, they are argued from. They are the starting point for all serious thinking, and, for that matter, for all unserious thinking. Every science and every academic discipline begins with them; every opinion about what is good or evil rests on them; every theory about human society or individual human happiness is founded on them.
A prime quality of a well-trained mind is the ability to know what first principles it has itself assumed as the starting points of its thinking and to perceive the first principles that undergird the ideas, opinions, and actions of those around them. Furthermore, a clear mind understands what is required to ground first principles reasonably. This kind of clarity of mind is more than just a nice academic accomplishment: the possession or the lack of it brings serious consequences.
Most of the angry arguments that so trouble our current social world are expressions of just this type of fundamental disagreement. Each side thinks the other to be stupid or unreasonable when discussing a specific matter because each has assumed a different set of principles that stand behind their opinions and give them coherence. Assumptions concerning what a human is, about what makes for human happiness, or about where we have come from and where we are going will precipitate all kinds of conclusions that touch on everything from politics to morality to seemingly trivial matters like fashion and music choices. All humans are philosophers, although they do not always know it. We all ground our opinions and our behavior on fundamental philosophic principles that we have picked up somewhere along the way.