Ethical Realism

October 24, 2010

Common Sense Assumptions vs. Self Evidence

I would like to know whether or not knowledge is possible, and whether or not it’s possible for a belief to be justified without observation. If observation is the only good reason to have a belief, then it’s not clear that we can have knowledge concerning mathematics, morality, or logic—and it might even be impossible to have knowledge about anything. Perhaps we can have good reasons to believe something based on common sense assumptions or self-evidence rather than observation. I will argue that that some beliefs are justified as “common sense assumptions” that are not self-evident, and they aren’t justified through observation. These assumptions are justified by embodying various intellectual virtues better than alternatives.

I will discuss the following:

  1. How observation fails to provide knowledge.
  2. Common sense assumptions
  3. Self-evidence

How observation fails to provide knowledge.

Observation is our ability to perceive or experience facts. We can see that we have hands, and we can drop objects repeatedly in an attempt to understand gravity. This is the main way that natural science tries to justify our beliefs about the natural world. The problem with observation isn’t that observation always fails to give us accurate information about the world; it’s that observation is incomplete—there are facts that observation can’t give us. Consider the following:

  1. Observation is impossible without assumptions. – When I see that I have a hand, I am actually assuming that I could have a hand before interpreting my perceptions as proving it. Instead of thinking that I am perceiving my hand, I could just think that I am seeing color splotches. Observation is said to be “theory-laden” because of the assumptions or hypotheses that are required to make sense out of observations. In this way observation is a form of circular reasoning. Our observations confirm our assumptions just as much as our assumptions confirms our observations. Circular reasoning doesn’t usually seem to lead to knowledge. If it did, then people who observed that some people are possessed by demons would actually know that people are possessed by demons; and people who see bigfoot know it exists; and people who see aliens visit the Earth know that aliens exist. There seems to be a missing ingredient to explain how observation provides knowledge.
  2. Mathematical facts aren’t provable by observation. – Yes, mathematics is relevant to observation. If you put one apple with another apple, then you have two apples. However, observation no more confirms mathematical truths than it can disprove them. Observation can’t prove that 1+1=2, and it can’t prove that 1+1=2 is false. Mathematics is indispensable to observation, but it isn’t proven to be true by observation. So, how do we know mathematical facts?
  3. Logical facts aren’t provable by observation. – Yes, logic is relevant to observation, but observation can’t disprove logic. If I observe that Newton’s theory of gravity fails to make the appropriate predictions, then it could very well be because Newton’s theory of gravity is false. When scientific theories fail to make predictions, we think that they must be false (in part) because they can’t be both true and false. That’s not an option we find acceptable. So, how do we know logical facts?
  4. An observation can’t confirm or disprove a scientific theory without an assumption. – In particular, we must assume that the future will resemble the past. Scientific theories make predictions because they assume that certain laws of nature exist and will continue to exist. You can’t observe that “the future will be like the past” because we can’t observe the future before it happens. We can see that the future was like the past in the past, but that doesn’t prove that the future will be like the past again unless we assume that it will—and therefore make use of viciously circular reasoning. So, how do we know that scientific theories are true? This issue is now called the “problem of induction.”

There are philosophers who believe that all knowledge is based on observation, but it is now pretty uncontroversial that observation requires assumptions. One way to explain how we can have knowledge (or rational beliefs) while using assumptions is to claim that knowledge (or rational beliefs) is allowed to use circular reasoning. This view is called “the coherence theory of justification” or “coherentism.” (Some have suggested a “coherence theory of truth,” but that is a more ambitious claim—that truth itself is nothing more than a belief with certain characteristics. This theory would require us to deny the correspondence theory of truth—that it’s true I have a nose because there’s something on my face that we call a “nose.”)

However, the coherence theory of justification is not our only option. In particular, I will discuss common sense assumptions and self-evidence.

Common sense assumptions

What I see as “common sense assumptions” has a history including G. E. Moore, coherence theorists, and pragmatists. What many called “common sense philosophy” was started by G. E. Moore, who realized that philosophers were denying the truth of things we know are true (in part) because they don’t know how we know they are true. It’s not easy to know how we know that 1+1=2, but it would be stupid to think an argument could prove it to be false. If anyone thought they proved that 1+1=3, we would think that the argument must be unsound. Moore thought that he could know that the Earth is older than he is, that he has hands, and that an external world exists other than himself; and he thought that other people can also know these things. Tom Baldwin describes G. E. Moore’s view of common sense in the following words:

Having set out these truisms, Moore then acknowledges that some philosophers have denied their truth or, more commonly, denied our knowledge of them (even though, according to Moore, they also know them) and he attempts to show that these denials are incoherent or unwarranted. These claims might seem to leave little space for radical philosophical argument. But in the last part of the paper Moore argues that his defence of common sense leaves completely undecided the question as to how the truistic propositions which make up the common sense view of the world are to be analysed; the analysis may be as radical as one likes as long as it is consistent with the truth and knowability of the propositions analysed. Thus, for example, he is content to allow that philosophical argument may show that a phenomenalist analysis of propositions about the physical world is correct.1

Moore’s understanding of “common sense” is compatible with self-evidence since he doesn’t tell us how his “truisms” are to be justified or “analysed.” In fact, Moore thinks that there are self-evident truths that could include various truisms.2 However, the conception of “common sense assumptions” that I am discussing here are not self-evident truths. They are more modest than that because they are unproven yet justified presuppositions.

I understand that the coherence theory of justification can be used to help justify our “common sense assumptions,” but I haven’t read many other people make that suggestion. The main idea of coherence is that two beliefs can mutually support each other through a sort of circular reasoning. The belief that the future will be like the past is supported by our past experiences, and our past experiences are also supported by the belief. However, coherence theories need not claim that coherence is the only way to justify a belief. Coherence theorists frequently appeal to various theoretical virtues, such as simplicity, empirical adequacy, and comprehensiveness.

Some people have suggested that pragmatism was a form of “common sense” philosophy. “Like some other philosophers, the pragmatists saw themselves as providing a return to common sense and the facts of experience and, thus, as rejecting a flawed philosophical heritage which had distorted the work of earlier thinkers.”3 Simply put, pragmatism is the idea that appropriately useful (or “pragmatic”) beliefs can be partially justified on the basis of their usefulness—or it’s often reasonable to believe something is true if it is useful to do so. It seems obvious that “the belief in an external world” is justified. I am not the only thing to exist in the universe, there are other people, animals, and objects—and yet the belief in an external world was not easy to prove using the traditional philosophical argumentation. The philosophical tradition seemed to have an overly skeptical understanding of observation or experience, and the pragmatists expanded our understanding to enable it to justify more beliefs. “In different ways, Peirce, James, and Dewey all argued that experience is far richer than the tradition had supposed, and that earlier philosophers were mistaken in their belief that we could identify ‘experiences’ or ‘sense data’ as separable constituents of cognition” (ibid.).4 John Dewey made this suggestion clear when he suggested that “[e]xperience is a process through which we interact with our surroundings, obtaining information that helps us to meet our needs” (ibid.).

How are common sense assumptions justified?

I suggest that we are justified in having certain beliefs, even if they aren’t proven to be true through observation or self-evidence or anything else. What exactly makes certain assumptions justified despite not being proven? These assumptions could be rational (or justified) insofar as (a) rejecting them is absurd, (b) it’s pragmatic to believe them, and/or (c) there is more (or equal) reason to believe them than reject them. For example, we believe that the future will be like the past (which philosophers call “induction.”) We can’t observe that this is true because we can’t observe the future, and it’s not self-evident. In fact, it’s quite possible that it’s false. Perhaps the future will be totally unlike the past. However, (a) thinking that the future won’t be like the past is absurd, (b) it’s pragmatic to believe the future will be like the past (we can’t do anything without that assumption), and (c) there is more reason to believe that the future will be like the past than the opposite (even though an attempt to prove that the future is like the past would require circular reasoning).

Why is thinking that the past won’t be like the future absurd? Because it’s a self-defeating belief. For example, if the future isn’t (usually) like the past, then language will probably change in unprecedented ways. We shouldn’t expect anyone to understand an essay or argument if language is likely to change in the future. So, arguing that the future won’t be like the past would be a waste of time. To even try to have a belief that the future won’t be like the past would be unproductive considering that the beliefs we have now would probably change in the future.

Common sense assumptions could also be justified by embodying non-observational, non-self-evident theoretical virtues better than the alternative beliefs. A justified common sense assumption could be more logically consistent, comprehensive, and simple than the alternatives. To assume that the future will be like the past is more consistent, comprehensive, and simple than to think it isn’t. To think it is like the past allows us to justifiably believe that bread is nutritious, cyanide is poisonous, and so on. If the future isn’t like the past, then bread could be poisonous, cyanide could be nutritious, and so on.

What I call “common sense assumptions” is similar to and compatible with coherentism, pragmatism, Karl Popper’s epistemological theory, and perhaps a form of fallibilist foundationalism.5 Consider how science operates—scientists have a hypothesis and try to disprove it. If the hypothesis makes risky predictions much better than the alternatives, then we take it to be true despite the fact that it might one day be “proven false.”6 Successful scientific theories seem like they are very accurate, even if they aren’t completely true. Although we want to say that there is evidence that our scientific theories are true, Popper argued that there can be no such evidence because of the problem of induction (i.e. we only assume that the future is like the past). Popper thought that we treat even our best scientific theory as a “working hypothesis.” We are rational to assume that a hypothesis is true until we have reason to think it’s false. Although Popper seemed to think that all scientific knowledge was presumptive in this way—I am merely suggesting that some justified beliefs are presumptive.


Why are people against presumptive justified beliefs? First, common sense is often wrong. Second, some people argue that all justified beliefs must be proven to be true—or at least be given evidence. Third, people argue that something isn’t true just because it’s useful. Fourth, common sense assumptions are nothing more than the coherence theory of justification. Fifth, why call these beliefs “assumptions?”

Objection 1 – Common sense is often wrong. People thought that the Sun orbits the Earth, that the Earth is flat, that ghosts meddle in human affairs, and so on—but we now know these beliefs are false. The human mind has various intuitive beliefs and instinctual assumptions that are at one time quite popular and uncontroversial, but could be eventually proven to be false.

My reply – The popularity or “intuitive” nature of a belief is not the main concern of common sense assumptions as I am using the term. I think that many truisms are obviously true and we have a difficult time telling people how we know they are true, but common sense assumptions as I use the term are justified. They are “common sense” because they tend to be boring and ordinary and no spectacular wisdom is required to know that they are true. The word “good sense” is sometimes suggested, but that term seems to suggest that a greater sort of wisdom has been achieved. I don’t think common sense assumptions require “good sense.” They require a minimal amount of “good sense” that would be better described as “moderately adequate sense.”

That’s not to say that common sense assumptions are always easily known. Some common sense assumptions could actually be profound discoveries. It seem likely that all common sense assumptions do require a minimum level of wisdom, and some might require quite high degrees of wisdom.

Objection 2 – All justified beliefs must be proven true. If a belief isn’t proven to be true through observation or self-evidence, then people could be justified to believe in bigfoot, unicorns, gods, aliens from space that visit the Earth, and teapots orbiting the sun. We can’t disprove the existence of these things, so it seems reasonable to shift the burden of proof to the believer—they should tell us why we should believe.

My reply – I don’t think common sense assumptions totally lack justification. A believer can and should give us reasons to agree with the belief. These beliefs are rational despite lacking traditional justification in the form of observation and self-evidence. These beliefs can be justified in other ways, through pragmatic considerations, coherence, comprehensiveness, and simplicity. Common sense assumptions seem to lack evidence, but we can have a reason to believe something other than evidence.

The reason that beliefs in unicorns is irrational is because the belief violates the theoretical virtue of simplicity (Occam’s Razor). The belief that unicorns are mythological is a simpler explanation than the belief that they are real. The belief in unicorns is an overly ambitious belief that requires a greater justification than is available.

Common sense assumptions is not merely a form of presuppositionalism. Many Christians defend their religion claiming that it can be rationally presupposed to be true without proof, and many philosophers have rightly criticized this theory. It is certainly not obvious that Christianity is justified through common sense assumptions or even through a coherence theory of justification. (Presuppositionalism might actually just require a coherence theory of justification, but it has had limited success.)

Objection 3 – Something isn’t true just because it’s useful. Some people argue that moral beliefs or beliefs in God “make them feel good.” This doesn’t seem like a good reason to have a belief because lots of false beliefs can make us feel good. For example, it might make a serial killer feel better to believe that “killing people is no big deal.”

My reply – First, pragmatically justified beliefs don’t necessarily override non-pragmatic considerations. If a belief is irrational because it is incoherent, then being pragmatic won’t help justify it. Second, I am not committed to saying that any belief is justified for being pragmatic. That is merely a suggestion. Common sense assumptions can be justified in non-pragmatic ways.

Objection 4 – Common sense assumptions are nothing more than the coherence theory of justification. Coherence theorists argue that some beliefs are justified despite not being self-evident or based on observation. This seems to allow for “common sense assumptions.”

My reply – There could be coherence theories that are compatible with common sense assumptions, but common sense assumptions don’t require us to accept a coherence theory of justification. First, I think we can have justified common sense assumptions without being justified through “coherence.” There are probably coherence theories that can accept that fact, but not all do. I would require a coherence theory of truth to have the following elements:

  1. Not all beliefs are justified merely through coherence. Observation could have a greater importance than many of our beliefs. We shouldn’t dismiss an observation just because it’s incompatible with many of our beliefs.
  2. Some common sense assumptions are justified. For example, my belief that the future will be like the past might be worth believing based on its theoretical virtues other than coherence.
  3. Some beliefs seem to be self-evident, and self-evidence should not be incompatible with a coherence theory.
  4. We shouldn’t reject highly justified beliefs just because they are incoherent with our other beliefs. For example, “1+1=2” is a justified belief even if many other beliefs don’t cohere with it—although it might give us some reason to doubt the other beliefs.
  5. Two beliefs can be equally justified (and highly justified) and we might have no way to decide one of them is false. It might make no sense to reject either belief. For example, suppose that a belief that killing people is usually wrong and a belief that morality doesn’t exist are equally and highly justified. It would be absurd to then conclude that killing people isn’t usually wrong.

Second, common sense assumptions could be foundational and coherence theories are not foundational. A form of fallibilist foundationalism seems to be compatible with common sense assumptions. It’s possible that all knowledge and justification ultimately rests on common sense assumptions. I would require a fallibilist foundationalism to have the following elements:

  1. We have justified common sense assumptions that are not in need of direct or indirect justification through a self-evident belief. It might be possible that some beliefs can be justified through self-evidence and something else, but my main point is that there is often no need to justify a common sense assumption through self-evidence.
  2. It isn’t self-evident that any beliefs are self-evident. It isn’t usually easy to know any belief is self-evident, and it’s possible that no beliefs are self-evident. If any belief is self-evident, such as “1+1=2,” then it takes some degree of wisdom to justify the fact that it is self-evident.
  3. Self-evident beliefs—if they exist—are fallible. We can think that a belief is true through self-evidence, but find out we were wrong.
  4. It could be that deep forms of knowledge are impossible. We might say that we “know” something when it’s sufficiently uncontroversial and “obvious,” but there might always be some room for doubt. It might be that justification for beliefs are never sufficient to absolutely prove something is true. This seems to be true of observation because we can’t know that observation is reliable through self-evidence.

Objection 5 – Why call these beliefs “assumptions?” It could be questioned why I call a proposed form of justified belief “common sense assumptions” instead of merely discussing “common sense justification.” After all, common sense beliefs are not “baseless assumptions” or merely presuppositional. They can embody actual theoretical virtues.

My reply – I want to make it clear that these beliefs lack a degree of certainty that self-evident beliefs enjoy to contrast them with self-evidence. I don’t think common sense assumptions are known for absolute certain even though we often say we “know” things, such as the fact that the sun will rise tomorrow. It might be possible to know a self-evident belief for certain.


Self-evidence is the ultimate sort of justification because it provides a non-circular form of justification. Additionally, self-evident truth doesn’t require that observation is reliable and it doesn’t require that the future will be like the past. The fact that 1+1=2 seems to be self evident. However, there are two problems with self-evidence. One, it’s incomplete. Two, it could be overly ambitious.

Objection 1 – Self-evidence is incomplete. Philosophers who believe in self-evidence almost unanimously agree that observation is also a reliable source of justification, but self-evidence isn’t require to justify what I call “common sense assumptions.” For example, it doesn’t explain why observation is reliable or how we know that an external world exists. Neither truths are controversial, but neither are self-evident. Additionally, the fact that there is an external world and that observation is reliable can’t be proven by observation alone. These truths seem to require what I call “common sense assumptions.”

I admit that some observation might provide us with self-evident truths. When I observe that the sun is up, it might be self-evident that the sun is up. However, this could only be true if observation is reliable—and we can’t know that from self-evidence alone.

Objection 2 – Self-evidence might be overly-ambitious. It’s not entirely clear that there are self-evident truths. Perhaps everything we call “self-evident truths” are actually “common sense assumptions.” Sure, we seem to know that 1+1=2, but we also seem to know that the future will resemble the past in many ways, and that there is an external world. It is silly to deny any of these “facts.” It could be that all of our “self-evident” beliefs are actually justified through common sense assumptions. The belief that 1+1=2 might be justified because (a) it’s absurd to deny it, (b) it’s pragmatic to believe it, and (c) there is more reason to believe it is true than that it’s false.

If we can somehow know that some beliefs are self-evident, then I have no problem with that. I am merely suggesting that many people aren’t convinced that self-evident truths exist, and common sense assumptions seem to easily replace the need for self-evidence.


Around a year ago I discussed how intuition (and our difficulty verbalizing our justifications) relates to various sorts of justifications in “Knowledge, Justification, and Theoretical Virtues” and “Objections to Moral Realism Part 2: Intuition is Unreliable” and and both self-evidence and common sense were discussed as potential forms of “intuition.” I have expanded the discussion here and defended common sense assumptions. As I said before, the existence of justified common sense assumptions is a neglected topic in philosophy and I know of no philosophers to directly discuss it. However, I must admit that common sense assumptions seem to be implied by Moore’s discussion of truisms, some forms of coherentism, and pragmatism.

Common sense assumptions provide us with a way to justify the reliability of observation and the belief that the future will be like the past. Moreover, many philosophers use self-evidence to explain our knowledge of mathematics, logic, and morality, but common sense assumptions might do the same job with less ambitious claims. Finally, self-evidence does not adequately explain how we know that observation is reliable or the future is like the past, but common sense assumptions can.

Update (3/14/2011): I didn’t fully explain or justify the fact that highly plausible uncontroversial beliefs are important to philosophy here, and I might write more about it in the future. However, I have written about this issue in more detail in “Common Sense Assumptions vs Self-Evidence.”


1 Baldwin, Tom. “George Edward Moore.” Stanford Encyclopedia of Philosophy. 22 October 2010. <>. Last updated in 2004.

2 Moore argues that “the kind of evidence, which is both necessary and alone relevant to such [an ethical] proof and disproof, is capable of exact definition. Such evidence must contain propositions of two kinds and of two kinds only: it must consist in the first place, of truths with regard to the result of the action in question – of causal truths – but it must also contain ethical truths of our first or self-evident class.” (Moore, G. E. “Preface.” Principia Ethica. 24 October 2010. Fair Use Repository <>. Originally published in 1903.)

3 Hookway, Christopher. “Pragmatism.” Stanford Encyclopedia of Philosophy. 24 October 2010. <>. Originally published in 2008.

4 Also, the reliability observation itself is difficult to justify in traditional philosophical thought. Observation is taken to be a primary source of justification by philosophers, but observation doesn’t justify itself. We can’t know that observation is reliable because we can’t observe the future. Additionally, it’s not self-evident that observation is reliable because it could very well be unreliable. We could be massively deluded about the reliability of observation.

5 Fallibilist foundationalism (also called “moderate foundationalism”) is the view that we can have justified beliefs that are neither inferential nor incorrigible—and fallibilist foundationalism could be compatible or even identical with “common sense assumptions” if we admit that some justified beliefs are neither self-evident nor inferential. Non-inferentially justified beliefs are called “properly basic.” (“Properly Basic” Wikipedia. 24 October 2010. <>.)

6 Of course, we might think a theory is “proven false” only to find out we were wrong.

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