Given the fact, as we have seen, that reason is the ultimate judge of truth and falsehood, a question immediately comes to mind. What is truth? This is a very fine question and it has a very simple answer. This short discussion will save you from wading through boring and confusing philosophical treatises on the subject, which rarely ever get to the correct answer anyway. Here goes...
We remember that "there is a way that things are." I will call this "reality." It's the definition of reality, after all, so no big stretch here. We also remember that this is another way to make the most fundamental claim that can be made about anything, and that is being, or existence, coupled with identity. We also remember that this is UNDENIABLE since to claim that there is not a way that things are is, in fact, to make a claim that there is a way that things are. Given this, and applying the law of identity, we can say that truth claims that correspond to (or match) reality are said to be TRUE. And truth claims that do not correspond to (do not match) reality are said to be FALSE. See how easy that is?
In epistemological circles this is called the correspondence theory of truth. We see, however, that this is no theory. Rather, it is a definition. Truth is that which corresponds to reality. Now many philosophers have gone off the rails (Hume and Kant, for two) by making the fundamental mistake of confusing ontology (Being) with epistemology (Knowing). This is called a category mistake. By analogy, it's like saying: That fabric sure feels loud. Feel is a word that we associate with the sense of touch and loud is a word that we associate with the sense of hearing. So we have made a category mistake when we do that. In English we say that we need to compare apples to apples and oranges to oranges. If we are making a category mistake we say that we are comparing apples and oranges and immediately we understand the error.
The confusion in (a lot of) philosophy is a confusion between ontology and epistemology. It's undeniable that truth exists. This is an ontological certainty. The problem comes in our epistemology. How do we know what that truth is? As it turns out, we have two tools for discovering what is true. Those tools are reason and sense experience. Now, ultimately, it doesn't get any better than this. There is NO BETTER WAY to determine what is true. But this need not plunge us into skepticism (all knowledge is uncertain) or agnosticism (reality is unknown or unknowable). In fact, I hope to show that what we call today the scientific method is well and truly the absolute best way to discover the truth about the material world. More on that later.
To say that truth is that which corresponds to reality is an exercise of pure reason. It's merely a definition. But that's not the end of the story. It's not the end of the story because not only is there a mental world of reason but there is also a physical world of sensation. We are directly aware of, or we can say that we directly experience the world of reason, with our minds. We also directly experience the material world, the world of sensation, by the use of our five senses. Those are: sight, hearing, touch, taste, and smell.
So in order to know what the material world is like we use what has come to be known as the scientific method. It's pretty simple, actually. It goes like this. (You can find other ways to say this, but essentially, this is it.)
Step 1. Gather, or observe data, or evidence. (This is an empirical [empirical means originating in, or based on, observation or experience] exercise. What qualifies as data is what can be seen, heard, tasted, smelled, or felt. In other words, the material world.)
Step 2. Analyze the data. (This is a rational exercise and it is accomplished by the mind.)
Step 3. Make an inference to the best explanation which means to propose a hypothesis that best explains the data. (This is also a rational act.)
Step 4. Gather, or observe more data. (Empirical exercise.)
Step 5. Compare to previous hypothesis. Modify as necessary to account for new (and old) data. (More reason.)
Step 6. Repeat until "all" of the data has been explained.
So we notice a few things about this method. The first is that its data come from the physical world. Science is empirical. It is about the physical world. The second is that the scientific method includes not only empirical observations but it also begins with the use of reason. Data without reason cannot be analyzed or interpreted or rendered meaningful in any way. Logically speaking, REASON MUST PRECEDE DATA. Without reason (mind) the very idea of data is incoherent. The third thing we notice is that reason is reason and data is data. (Law of Identity.) By that I mean to say that facts are facts. They are neither true nor false. They just are, or they are not. They are not subject to reason in their existence because they are a part of the physical world. The interpretation of the facts (or data), on the other hand, is an exercise in reason. And unless that reasoning is done correctly, then wrong answers are likely to result. I say likely because it is possible to reason incorrectly and still get a true answer by luck.
All cats are mammals.
All snakes are reptiles.
Therefore: All frogs are amphibians.
The conclusion is true but the syllogism is invalid. (For reasons I won't go into here.) So we have reasoned incorrectly with true premises and managed to get a correct answer. This is not the real beauty of deductive reasoning (reasoning from the general to the particular). The real beauty of deductive reasoning is that IF one has true premises AND a valid (structurally correct) form of argument, THEN the truth of the conclusion is CERTAIN. By certain, I mean 100% of the time. Necessarily true. Impossible to be false. This will be so important in so many ways that I must belabor it one more time. If a deductive argument is valid (structurally correct) and the premises are true (the argument is sound), THEN the conclusion HAS TO BE TRUE. It is impossible for the conclusion not to be true. I will demonstrate this later. Meanwhile, back to the scientific method...
To reiterate the most important observation we make about the "scientific method." It starts with reason and data. This is fundamental and probably the most important observation about the scientific method that we can make. It starts with reason and data. This means that we reason from evidence to conclusion. Let me say that again. We reason from evidence to conclusion. If we start with something besides reason and evidence (for example, a metaphysical assumption that is not a first principle, or directly deducible from first principles) THEN WE ARE NOT "DOING" SCIENCE. We are engaged in (poor) philosophy or (poor) religion or (poor) theology or something else but it is NOT SCIENCE. Nor is it intellectually honest in any way, shape, or form. If we want to have intellectual integrity we must start with reason and the evidence and go wherever that takes us. Wherever that may be. Whether you like where that is going or not. Intellectual integrity DEMANDS that we do not reason improperly or lie about, or misrepresent, our evidence. The two major crimes against Truth are reasoning incorrectly and corrupting the evidence. If we do either one of these things, particularly if we do them deliberately, we have forfeited the right to be taken seriously and are deserving of contempt. In my opinion, anyway. I will have much more to say later.