Ethical Realism

May 31, 2011

Three Forms of Evidence

An argument uses premises to reach a conclusion, but we can’t just accept that every valid argument proves the conclusion to be true. If an argument has a valid form, we need to know that the premises are true before we can know the conclusion is true. We rarely know for certain that the premises of an argument are true. Instead, we do our best at justifying the premises. One way to do this is to provide evidence—reasons we should believe something to be likely true or accurate. Many people equate “evidence” with “observation,” but there could be other reasons to accept beliefs as well. I will discuss three types of evidence:

  1. Observation
  2. Introspective experience
  3. Intuition
  4. Noninferential justification


Observation or “empirical evidence” is evidence based on experience. What we perceive with the senses is observation. For example, I can see a cat on a mat, feel my hands, taste my food, and hear a barking dog. Observation is the main source of scientific evidence, but it’s also a source of much of our common knowledge. Consider the following argument:

  1. Socrates is a man.
  2. All men are mortal.
  3. Therefore, Socrates is mortal.

This argument is valid, but is it sound? Do we know that the conclusion is true because the premises are true? I think so. How do we know Socrates is a man? There were eyewitnesses who described him and his behavior. This information was recorded in texts. How do we know all men are mortal? Every man we have ever observed has died. There are no men we know of who have lived longer than 150 years. Both premises are justified using observation.

Not all observation is direct. Sometimes scientists use tools, such as microscopes, to enhance our ability to observe the world around us. In the argument above we rely on the observations of others. Additionally, we often rely on the experiences of others. I haven’t known enough men to know that they are all mortal on my own, but we figure that if there were any immortal men, then they would have been discovered by now; and such a discovery would have been shared with the rest of us.

Observation is “theory-laden” in the sense that all observation must be interpreted and such interpretations are based on assumptions. For example, my observation that I have two hands is based on the assumption that I’m not sleeping, that an external world exists other than myself, and that the experiences I have are best understood with the assumption that my two hands are causing them. Without interpretation I would have experiences of colors, movement, feelings, and so on; but I couldn’t know that I have two hands without assuming that such colors, movement, and feelings are based on an external reality containing solid objects and so on.

Introspective experience

Introspection—an examination of what it’s like to have experiences—involves observations without a concern for objects outside of ourselves. I have introspective experiences of my thoughts and feelings, and these experiences aren’t merely based on sight, sound, touch, taste, or smell. These experiences aren’t of anything outside of myself or part of an external world. We tend to assume these experiences are “within the mind” and are “psychological.”

Introspection gives us access to understand qualia—the “what it’s like” of our experiences. My experience of pain is a clear example of a qualia. One thing that’s important about pain to me is what it’s like to experience it.

Some of my introspection seems quite unlike perception. Sometimes I have thoughts that aren’t put into words and don’t seem like anything I can perceive with the five senses. However, many introspective experiences are related to perception. When I see a green frog, I think I’m experiencing something outside of myself that’s part of an external world using my eyes. However, the green color of the frog looks a certain way to me that might not be part of an external world. Each color has a qualia, a way it looks to us that’s not merely wavelengths of light reflecting off of objects. The qualia of each color is what differentiates the look of each other color and many people have a “favorite color” based on how it looks to them.

Consider the following argument that makes use of introspective experience:

  1. Pain is bad.
  2. We usually shouldn’t make bad things happen.
  3. Kicking and punching people causes pain.
  4. Therefore, we usually shouldn’t kick and punch people.

We know that “pain is bad” because we experience it that way. The second premise that “we usually shouldn’t make bad things happen” is more difficult to justify, but it’s a common assumption among people. The third premise that “kicking and punching causes people pain” is quickly discovered by people through observation after they are kicked and punched by others.


We often say that a belief is “intuitive” (e.g. solid objects exist) or counterintuitive (e.g. solid objects don’t exist), and what’s intuitive is often taken to be justified and what’s counterintuitive is taken to be unjustified. We often call intuitive beliefs “common sense” but not all intuitive beliefs are “common knowledge.” Intuitive justification can require maturity and understanding that most people fail to attain. Prejudice, having a hunch, or “women’s intuition” is not intuition of the philosophical variety, although we might often confuse them with the philosophical variety.

What exactly it means to say something is “intuitive” isn’t entirely clear, and there are at least three different forms of intuition: (1) the justification for a belief that’s hard to articulate in words, (2) assumptions we have found successful, and (3) noninferential justification. I will discuss each of these.

1. Justification that’s difficult to articulate

It’s often impossible to fully articulate why our beliefs are justified. We think we know some of our beliefs are true with a high degree of confidence, even if we can’t fully articulate how we know the belief is true, and even if we can’t fully justify our belief to others using argumentation. For example, “1+1=2” is intuitive, but it’s hard for many of us to prove it’s true and fully explain how we know it’s true. Intuitive beliefs could be based on any form of evidence: Observation, introspection, successful assumptions, noninferential justification, etc. What we know from these sources of justification are not necessarily easy to fully understand or communicate to others.

Although intuitive beliefs are difficult to prove to be true through argumentation, many philosophers try to justify them using arguments. This might actually just prove to other people that they share our intuitions. Consider the following intuitive argument:

  1. Imagine that what always happens in the past isn’t likely to happen in the future because the laws of nature will change. In that case we have no reason to think (a) the sun will rise tomorrow or (b) eating lots of fatty foods tomorrow will be unhealthy.
  2. However, we know that the sun will probably rise tomorrow and that eating lots of fatty foods tomorrow will probably be unhealthy.
  3. Therefore, what always happened in the past will likely happen in the future because the laws of nature will probably stay the same.

We often generalize about what happened in the past to predict the future, but it’s difficult to prove that the future will ever be like the past—even though we often think we know it will be with a high degree of confidence. The first premise emphasizes the high confidence we have that the sun will rise tomorrow and eating lots of fatty foods tomorrow will be unhealthy to show how counerintuitive it is to believe that what always happened in the past probably won’t be like the future because it requires us to reject certain beliefs we think we know are true.

2. Successful assumptions

Some intuitive beliefs could be successful assumptions similar to how scientists use provisional “working hypotheses” that seem to explain our observations until they are proven false. (e.g. Scientists assumed that the Sun revolves around the Earth at one point.) In that case it’s hard to explain how justified our intuitive belief is because it’s hard to explain to people all the ways the belief has proven to be successful. It could be that our assumption that the laws of nature will still be the same in the future is a successful assumption of this kind, and it seems highly successful. Rejecting this assumption would make living our lives impossible. We could never assume that food will be nutritious or that money could still buy goods tomorrow, but we continually find these assumptions to be successful.

If a belief is a successful assumption, then we can explain how the belief is justified based on successful risky predictions, the lack of viable alternatives, and the possibility of attaining counter-evidence. Our assumption that the future laws of nature will be the same has enabled us to make every successful risky prediction we’ve ever made, the rejection of such a belief seems absurd rather than a serious alternative, and we could imagine counterevidence against such a belief (e.g. if the law of gravity stopped functioning tomorrow).

On the other hand the assumption that I can’t find my keys because a ghost moves them lacks support from risky predictions and it fails to be as viable as alternatives (e.g. maybe I just forget where I put them).

3. Noninferential justification

Noninferential justification is evidence that we can understand without an argument. One possible source of noninferential justification is “self-evidence.” Some self-evident beliefs could be true by definition, such as “all bachelors are unmarried” and others could be justified based on the concepts involved. Perhaps anyone who understands what the concept of pain is will then understand that “pain is bad.” Many philosophers agree that what’s true by definition can be known noninferentially, but it’s much more controversial to think that conceptual knowledge can be justified using noninferential evidence beyond our definitions.

Noninferential justification is notoriously difficult to communicate to other people, but many mathematical concepts like “infinity” do seem to be plausibly understood in noninferential ways.


Arguments without evidence are not informative. Whenever we provide arguments, we need to consider how we know something is probably true or justified. If this is difficult, then it is likely that our conclusion is either unjustified or that we have intuitive evidence for it.

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