St. Thomas Aquinas, citing Aristotle, once wrote : “a small error at the outset can lead to great errors in the final conclusions.” What he means is that given the nature of reason, if any one of your premises is mistaken, no matter how trivial it may seem to your overall project, your conclusions may turn out to be wrong, very wrong.
A great example of what St. Thomas means can be found in a TED talk that my brother Jim suggested I watch. In “What Explains the Rise of Humans ?”, Professor Yuval Noah Harari  argues that the homo sapiens dominance of the earth is best explained by the human imagination’s ability to construct certain “stories” about “fictional entities” that provide the means by which we can, in large numbers, cooperate with one another. Among these fictional entities are God, human rights, and the value of paper money.
Although Professor Harari is an engaging speaker and his talk rhetorically attractive, the philosophical credentials of his theory left me with more questions than the theory has the resources to answer.
Let’s begin by asking this question: How does Professor Harari know that these “stories” about the divine, natural rights, and a nation’s currency are “fictions”? He does not say. All he does is assume that the correct account of reality is materialism, the belief that the only things that are “real” are those physical things that are subject to quantifiable measure.
As he says about the nature of human rights:
Human rights, just like God and heaven, are just a story that we’ve invented. They are not an objective reality; they are not some biological effect about Homo sapiens. Take a human being, cut him open, look inside, you will find the heart, the kidneys, neurons, hormones, DNA, but you won’t find any rights. The only place you find rights are in the stories that we have invented and spread around over the last few centuries. They may be very positive stories, very good stories, but they’re still just fictional stories that we’ve invented.
So it turns out that because human rights (not to mention, God) cannot be detected by the instruments and methods of the natural sciences, they are not part of “objective reality.” But like the country singer Johnny Lee, who once sang  of his vain search for love in “single bars” and with “good time lovers,” Professor Harari is looking for rights in all the wrong places. He is, as the philosopher Edward Feser puts it , like “the drunk who insists on looking for his lost car keys under the lamp post, on the grounds that that is the only place where there is enough light by which to see them.”
Where then should we “look” for rights? We need not go further than Professor Harari’s own lecture. By offering an account of the rise of humans that he believes is correct, he is implying that those who disagree with this account are mistaken. Assuming that the purpose of argument, as well as the use of evidence in support of an explanation, is to arrive at the truth or something approximating the truth, it follows that the mistaken person has no right to claim that he is correct.
It also follows from this that a person who ignores evidence, good reasoning, and thoughtful reflection, while embracing wishful thinking, fallacious reasoning, and thoughtless meandering, is wronging himself. Yet to make such a judgment one must know the ends to which the human person is ordered.
But such ends, or final causes, cannot be detected by the instruments and methods of the natural sciences. If you cut open a human being, as Professor Harari would put it, you cannot see the goods to which the person is ordered. If that is what makes such goods not part of “objective reality,” however, then the practitioners of the scientific enterprise itself are bereft of any grounds by which to condemn ignorance and extol wisdom, two judgments whose veracity depends on the “fictional story” of an immaterial reality, a human being’s form . After all, you cannot know that a being comes up short in the use of any of its natural powers unless you first know the sort of thing that it is. Thus, we say a blind person lacks sight while a sightless stone lacks nothing.
The laws of logic are also central to the scientific enterprise. That is, in order to engage in a scientific inquiry one should reason well, which means that one should not violate the laws of logic. But the laws of logic are not material entities that one can find by cutting anything open, let alone a human being. In fact, the relationships between an argument’s premises and terms are logical, not spatial, which means that they are not physical objects. Consider a valid argument form, modus ponens:
If P, then Q
This is a valid form, not because the two premises somehow together physically cause the conclusion, as a cue ball moves the 8-ball when they touch. Rather, as a matter of logical necessity, the conclusion is entailed by the premises.
That relationship is not physical, though it seems just as real and part of “objective reality” as the relationship between the two billiard balls or what one sees when one cuts open a human being. So, we have yet another reason to reject Professor Harari’s materialism.
Here’s the point: if someone offers you a theory of reality that excludes what seems to be obviously true, it’s probably a good idea to be skeptical of the theory rather than to doubt common sense. For it is, ironically, our common sense – what we pre-reflectively believe about the good, the true, and the beautiful – that makes theory-making, even bad theory-making, possible.