There are at least 4 core assumptions to realism:
Law of non contradiction
Analogical use of language
Basic reliability of the sensory perception
Law of causation
Law of Non Contradiction
These core assumptions were part of the epistemological system of Thomas Reid and published in his “An Inquiry into the Human Mind: On the principles of common sense” in 1818, and “Essays on the Powers of the Human Mind” in 1827. These were posthumous publications (Reid lived from 1710 until 1796). Reid was a realist. He was writing in response to Hume. I came to learn these 4 core assumptions from R.C. Sproul’s Defending your Faith Series.
The law of non contradiction is simply that something cannot be and not be in the same way at the same time. It really prevents skepticism. Not being skeptical about a claim, that is a healthy part of dialogue and as we will see an important part of knowledge. It presents against universal skepticism - after all, how you claim “to know” that nothing can be known? Or claim that nothing is true? If nothing is true, then by non contradiction your statement “nothing is true” is called to question. So, to be a universal skeptic you need to disagree with the law of non contradiction. For KBP, I am starting with the assumption of non contradiction. To be honest I am not completely sure why - an empiricist would say because I have never observed (or heard it to be observed) that something can be and not be in the same way at the same time; a rationalist would say that I can deduce the law of non contradiction in my mind and know it is true without empirical observations. For me, this is something I am willing to accept and not go further into why I accept it. I think it is reasonable, if I need to work on that - please let me know.
The clinical examples of the law of non contradiction at work are pervasive. We know that something cannot be and not be at the same time in the same way. Someone cannot be healthy, and not be healthy, in the same way at the same time. You can discuss different ways of being healthy, someone can be physically healthy and mentally not healthy, but that does not violate non-contradiction because it is a different “way” of being healthy. As we reason in clinical scenarios we are often doing tests where it is either positive (and we believe something to be true and something else to be false); or negative where we then believe something to be true and something else to be false) - that is because we are, in part, relying on the law of non contradiction. When we test the reliability of any measure we are assuming that there is a truth to the measure. If there is a discrepancy between raters we assume that is because one rater made an error (or they both made an error) because it cannot be true that the patient both has crackles and does not have crackles “in the same way at the same time.” If claimed that they do - we look for reasons - the first reasons we should look is “the same way” - did each rater have a different idea of what a crackle is? or different hearing thresholds for crackles? Next we can consider time, was there enough time between measuring that the patient’s status could have changed? I think you get the point - much of our clinical reasoning is predicated on and sustained by a belief in the law of non contradiction. As we move ahead I will be sure to point out when it is used.