1.2.1 Sources of Evidence

Even though philosophy is not an empirical science, philosophical claims require evidence, and philosophers ought to have reasons for the claims they make. There are many different types of philosophical evidence, some of which follow.

History

A basic but underappreciated source of evidence in philosophy is the history of philosophy. As we have already seen, philosophical thinking has its origins around the world, from the beginning of recorded history. Historical philosophers, sages, natural philosophers, and religious thinkers are often a source of insight, inspiration, and argument that can help us understand contemporary philosophical questions. For instance, the Greeks recognized early on that there is a difference between the way we use language to talk about things, with generic terms that apply to many different things at the same time (like cat, tree, or house), and the things as they actually exist—namely, as specific, individual beings or objects. Philosophers ask, what is the relationship between the general terms we use and the specific things that exist in the world? This sort of question is a perennial philosophical question. Today’s philosophers have their own response to this sort of question, and their answers often respond to and are informed by the historical treatment of these issues.

While you may expect questions about the natural world to change over time (and certainly they have changed due to scientific progress), questions of morality and social organization do not change as much. What constitutes the good life? How should communities be organized to benefit all the members of that community? These sorts of questions stay with us throughout time. In the United States, it is common for political leaders to appeal to the “founding fathers” of the US Constitution. People like Thomas Jefferson, Benjamin Franklin, and George Washington were heavily influenced by early modern European philosophers like John Locke, Jean-Jacques Rousseau, and Thomas Hobbes. In similar fashion, the current Chinese leader, Xi Jinping, is fond of reading and citing the foundational philosopher Confucius. Most of Xi’s addresses include quotations from Confucius, and Xi stresses the importance of reading classical Chinese philosophers (Zhang 2015). For Chinese political leaders, Confucius provides an important reminder of the role of virtue and a sense of belonging among the Chinese people. There is a widespread belief among the Chinese political class that their intellectual heritage is an important factor in their contemporary political success, in much the same way as American political leaders trace their success back to the founding fathers. Given the influence of philosophy on world history, it is worthwhile to engage with the writings of past philosophers to inform our understanding of pressing philosophical questions of today.

Intuition

One of the hallmarks of philosophical thinking is an appeal to intuition. What philosophers today mean by intuition can best be traced back to Plato, for whom intuition (nous) involved a kind of insight into the very nature of things. This notion has had religious connotations, as if the knowledge gained through intuition is like catching a glimpse of divine light. But intuition does not have to involve faith. René Descartes defined intuition in the following manner: “By intuition [I mean] . . . the conception of a clear and attentive mind, which is so easy and distinct that there can be no room for doubt about what we are understanding” (Descartes 1985, 14). This concept of intuition is clearest in mathematical examples. Importantly, it is quite different from the way that many people use the word intuition today to mean something like “gut feeling” or “hunch.” When philosophers talk about intuition, they mean something much more definite. Consider the equation 2 + 2 = 4. Examine the equation in your mind. Could it possibly be false? So long as we operate under the assumption that these numbers represent counting numbers, it seems impossible that this equation could be false. More than that, there is a kind of clarity and certainty about the equation. It is not just that you have learned 2 + 2 = 4 by habit. You could easily perform the counting operation in your head and verify that the answer is correct. The truth of this mathematical sentence is so clear that if it turned out to be wrong, you would have to give up core beliefs about the nature of numbers, addition, and equality. This kind of clarity is a paradigm of intuition.

Intuition operates in other realms besides mathematics, such as in the use of language. For instance, it is obvious that a three-legged stool has three legs or that the tallest building is taller than any other building. These statements are true in an obvious way that is similar to the mathematical sentence above. We can branch out further, to say, for instance, that a camel is a mammal. We might intuitively know this statement is true, but we may also recognize that we are on slightly less certain ground. After all, whether a camel is a mammal is based on some understanding of the anatomy of a camel as well as the biological classification system that assigns animals to different classes. So the definition of camel as “a mammal” is not the same as “a three-legged stool has three legs.” Here, we can see that some statements are intuitively true by virtue of their definition. Others are intuitively true by virtue of some mental operation that we can perform very easily. Still others are intuitively true in that they rely on a body of knowledge that is commonly accepted and foundational for our understanding of the world.

There are many other places outside of pure linguistic analysis and mathematics where intuitions are helpful. Consider morality: the proposition that “it is better to be good than to be bad” may seem similar to the statement that “a three-legged stool has three legs,” but the former introduces the words good and bad, which are fraught terms that produce disagreement among people. Nonetheless, while it may be difficult to agree on what constitutes “good” or “bad,” everyone probably recognizes that whatever is good ought to be better than what is bad. That seems intuitively true. On this basis, we might imagine that there are intuitive truths even in morality. As we gain confidence in the ability of intuition to reveal truth, we might be tempted to extend intuitions even further. However, when intuitions extend into areas where there is no consensus on what is true, we have to be cautious. At that point, we might be using the term intuition to stand in for belief or perspective. Such “intuitions” do not have the same force as the intuition that 2 + 2 = 4. It is not always easy to distinguish between intuitions that are certain and evident and those that are mere feelings or hunches; recognizing that distinction is part of the practical know-how philosophers try to develop.

Common Sense

We ought not to neglect a third source of evidence in philosophy, namely, common sense. The idea of common sense is frequently used to describe a basic set of facts or common knowledge that any adult human being ought to possess. But common sense is rarely defined. When philosophers talk about common sense, they mean specific claims based on direct sense perception, which are true in a relatively fundamental sense. In other words, philosophical champions of common sense deny that one can be skeptical of certain basic claims of sense perception.

Famously, early-20th-century British philosopher G. E. Moore argued that a perfectly rigorous proof of the external world could be given by simply making the appropriate gesture toward his right hand and saying, “Here is one hand.” So long as it is granted that the sensory perception of a hand is evidence of the existence of a hand and that there is such a thing as a hand in the external world, then it must be granted that there is an external world. Such an argument trades on the idea that knowledge of the existence of one’s own hands is something that does not need further proof; it is something we can know without proof. This idea is not something that all philosophers accept, but it is, in many cases, an important source of evidence in philosophical inquiry. At a certain point, it may be necessary to stop demanding proofs for the things we can plainly see, such as the fact that this is a hand (as we hold a hand in front of our faces and examine it). Common sense may be questioned by further philosophical interrogation, but the common-sense philosopher may respond that such interrogation is either unnecessary, excessive, or misses the point.

Experimental Philosophy

Experimental philosophy is a relatively recent movement in philosophy by which philosophers engage in empirical methods of investigation, similar to those used by psychologists or cognitive scientists. The basic idea motivating experimental philosophy is that philosophers use terms and concepts that can be tested in a laboratory. For instance, when philosophers talk about free will, they frequently cite the idea that free will is necessary to assign moral responsibility; thus, moral responsibility is one reason to believe in the existence of free will. Consequently, you might wonder whether most people do, in fact, believe that the existence of free will is necessary to assign moral responsibility. This claim can be tested, for instance, by posing problems or scenarios to research subjects and asking them whether the absence of free choice removes moral responsibility. Similar strategies have been applied to causation, philosophy of biology, consciousness, personal identity, and so forth. In these areas, philosophers use experimental methods to find out what average people think about philosophical issues. Since common sense and intuition are already a source of evidence in philosophical reasoning, it makes sense to confirm that what philosophers ascribe to common sense or intuition aligns with what people generally think about these things.

Such experimental research is subject to many of the same issues that confront experimentation in the social sciences. These studies need to be replicable and ought to fall within a psychological or biological theory that helps explain them. When philosophers tread into experimental philosophy, they behave a lot more like scientists than philosophers, and they are held to the same rigorous standards as other researchers in similar experimental disciplines.

Results from Other Disciplines

The relevance of experimental methods for philosophy suggests a broader source of evidence for philosophical claims, namely, the results of scientific disciplines. When philosophers make claims about the natural world, they ought to be aware of what the natural sciences say. When philosophers make claims about human nature, they ought to be aware of what biology and the social sciences say. As we have already seen, there is an important difference between philosophical investigation and these various disciplines. Yet, given that philosophers attempt to gain some understanding of truth as a whole, they ought to welcome evidence from other disciplines that can help them better understand portions of that whole truth.

Table 1.1 summarizes these different types of philosophical evidence.