A belief is not an assumption, otherwise people would use the term 'assumption' more frequently, and 'belief' less frequently, than those terms are used colloquially.

In a reductio ad absurdum proof, you guess A, reason until you arrive at a contradiction, then you conclude not A.

So an assumption is a guess that some proposition is true, for use in a reasoning event.

The two terms certainly are similar, so what is proper definition of 'belief', given that they aren't identical in meaning? Additionally, does the act of belief go against reason, that is, is it irrational?

  • I believe it isn't irrational. Commented Jun 19 at 14:15
  • The Sun has risen every day of your life so far, so you believe it will rise tomorrow. Do you think that's irrational?
    – Barmar
    Commented Jun 19 at 14:31
  • Meta-example: Is it rational to believe that you'll get helpful answers by posting in Stack Exchange?
    – Barmar
    Commented Jun 19 at 14:32
  • @Barmar, I don't usually get helpful answers on stack exchange, but I have. So you don't know unless you try.
    – lee pappas
    Commented Jun 19 at 15:28
  • Touché, you got me there.
    – Barmar
    Commented Jun 19 at 15:35

4 Answers 4


What is the definition of 'belief'... ?

A belief is a proposition that you regard as true. (And if justified and true, many would call such beliefs knowledge; but of course not all beliefs are knowledge.)

You're right that we can take on assumptions without regarding them to actually be true. Assumptions are not necessarily beliefs. We can assume things for the sake of an argument, or to see where a line of logic will lead, or to act under uncertainty, etc.

does the act of belief go against reason, that is, is it irrational?

There is no necessary connection between beliefs and rationality. Some beliefs are rational or justified. Others are irrational or unjustified.

  • there are things I know. I know I exist. I regard it as true. It seems wrong to say I believe I exist. Your answer seems to equate knowledge and belief. If you know X, you are not uncertain that X, but if you believe X, you are uncertain that X. Therefore knowledge and belief are different. Certainty and uncertainty are states of mind, you can be certain of X, and be wrong.
    – lee pappas
    Commented Jun 19 at 15:21

A belief is any proposition you hold to be true, which implies that you will rely on it to make decisions.

As always, think of designing a robot. The robot needs an internal model of the world in order to navigate that world. This model may take the form of a list of propositions in the robot's memory, or it may be expressed as such a list. The propositions in the list, which the robot relies on to navigate the world, are the belief set of the robot.

It is the same for humans. We have an internal model of the world, which we may usefully imagine as a set of propositions. The propositions that we rely on are our beliefs.

Beliefs need not be certain. You can have a belief that you're only 90% confident in. Or maybe even 60%. By convention, we'd usually say that to believe something, you need belief greater than some vague cutoff level (which might be 50%, 60%, 70%, depending on context). If you put only 40% credence in a proposition, we say you do not believe it (though we could say you believe it may be true).

  • Yes. I am fond of the saying, "You know nothing until you can prove it." So, beliefs would be unproven ideas. Also, one can be certain and be incorrect.
    – Scott Rowe
    Commented Jun 19 at 0:58
  • 2
    @ScottRowe I'd say proven ideas are also beliefs (and it's irrational to have 100% confidence in anything, even if it's mathematically proven, due to the however-slim possibility of logical mistakes.)
    – causative
    Commented Jun 19 at 1:00
  • "It's beliefs all the way down" ha ha Why do people set so much store by beliefs then? Mystifying.
    – Scott Rowe
    Commented Jun 19 at 1:01
  • @causative I see it the same way. Another point to add is that if someone had 100% certainty, then they couldnt become more certain, so even the production of more evidence/proof/replication wouldn't increase certainty - which seems absurd. Commented Jun 19 at 1:50
  • 1
    @MichaelCarey That depends on your conception of what certainty is. Like is it the heap of evidence? Well then there is no certainty you just approach it asymptotically. While for example in math "certainty" is an internal consistency with the axioms, so millions of examples can't produce certainty, but once you've reached it you don't need any more examples, they just exemplify not prove anything.
    – haxor789
    Commented Jun 19 at 9:11

What is the definition of 'belief'

A western belief is, according to the SEP, the

attitude we have, roughly, whenever we take something to be the case or regard it as true

You will notice that this is intentionally vague, especially it does not say anything, or require any measure of "objective" truth (whatever that may be, it is a complicated concept in its own).

Also you will notice it is not related to anything "mystic" by default. You can believe things that are truly physical, everyday things that are not under any doubt at all. E.g. "I believe it may rain later" is fundamentally the same as "I believe the moon is made from cheese" or "I believe there is a afterlife", albeit of course different in the details.

It is a precursor, but not the same, as knowledge (which is often, but not always, thought to be "Justified True Belief", with the conflict mostly again in the word "True", which suggest objective truth, which is complicated).

is it irrational to have one?

It is impossible to not have one; it is a regular feature of our brains, it is in our nature. You can describe it and discuss its implications, but not try to not have it.

It may or may not be irrational to think that a belief is also true in the clear absence of evidence (whatever that is, again! There are many different opinions/interpretations about all these terms).


A belief is not an assumption, otherwise people would use the term 'assumption' more frequently, and 'belief' less frequently.

No. Trees are plants, and calling them trees all the time does not mean trees are not plants.

Beliefs could thus be defined as special assumptions, e.g. long lasting and stable assumptions held without immediate practical necessity.

See https://plato.stanford.edu/entries/belief/

Anglophone philosophers of mind generally use the term “belief” to refer to the attitude we have, roughly, whenever we take something to be the case or regard it as true. To believe something, in this sense, needn’t involve actively reflecting on it: Of the vast number of things ordinary adults believe, only a few can be at the fore of the mind at any single time. Nor does the term “belief”, in standard philosophical usage, imply any uncertainty or any extended reflection about the matter in question (as it sometimes does in ordinary English usage).

Or here https://iep.utm.edu/aim-of-belief/

It is often said that belief has an aim. This aim has been traditionally identified with truth and, since the late 1990s, with knowledge.

Here https://brewminate.com/epistemology-the-history-and-philosophy-of-knowledge-and-belief/

One of the core concepts in epistemology is belief. A belief is an attitude that a person holds regarding anything that they take to be true.

Also Wikipedia https://en.m.wikipedia.org/wiki/Belief

A belief is a subjective attitude that a proposition is true or a state of affairs is the case. A subjective attitude is a mental state of having some stance, take, or opinion about something.

Additionally, does the act of belief go against reason, that is, is it irrational?

It would be irrational to hold a belief that contradicts knowledge or to hold mutually contradicting beliefs at the same time. Arguably holding strongly unnecessary beliefs against Occam's razor can be be another way to have irrational beliefs, and there may be more ways. But plenty of beliefs are rational and necessary for reasoning.

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