What philosopher said: "Whether I see something or not, if I don't believe in it, it doesn't exist."?

It’s my understanding that it’s still debated among scholars whether Descartes held this belief.

It sounds like the response for an agnostic.

Solipsism, the philosophical idea that only one’s mind is sure to exist, was first recorded by the Greek presocratic sophist, Gorgias (c. 483–375 BC)

https://en.wikipedia.org/wiki/Solipsism

The closest I can think of is the pragmatic philosophy of William James, which says that the truth of something depends on the practical value of believing in it.

I cannot think of any serious philosopher who has said this. It is a common misunderstanding of some philosophers (e.g., Descartes and Berkeley), but both are careful to point out that it is not what they mean (and is nearly the exact opposite of what they are saying).

Note that not even solipsism goes so far as to make the claim that belief determines existence. It is merely a claim about what can be known to exist (and, in some cases, a claim about what does exist).

It seems, then, that you have misremembered or misinterpreted some bit of philosophy that you read or heard somewhere.

@Tropical_Willie ”It sounds like the response for an agnostic.” Very interesting thought.

@SavoirFaire: And William James?

@SavoirFaire ”...both are careful to point out that it is not what they mean”

The argument I’ve heard regarding Descartes is that the part where he argues that “it’s not what he means” is actually disingenuous and is only done to avoid being charged as a heretic by the Church.

@SavoirFaire: Also, about agnosticism?

@luigirovatti James never says that the existence of something depends on our belief in it. The pragmatic theory of truth is not about what exists. It is about when it is appropriate to use the predicate “is true.” For the pragmatist, it is appropriate to call a statement true when it fits in with our beliefs and observations in a way that is useful. Both parts are important here: “fits in with our beliefs and observations” is supposed to prevent us from declaring something to be true just because we want it to be, while “in a way that is useful” is supposed to reflect the functionality of making true statements (we are interested in what is true because it helps us do thinks like make decisions and predictions). It is also worth noting that, unlike other pragmatists, James is more interested in the question of why we care about truth than the question of what exactly truth is.

As for agnosticism, “it sounds like the response for an agnostic” is ambiguous between “it sounds like the response of an agnostic” (i.e., “it sounds like what an agnostic would say”) and “it sounds like the response to an agnostic (i.e., “it sounds like a rejoinder to agnosticism”). I interpret @Tropical_Willie as meaning the former rather than the latter. In any case, the whole point of agnosticism is to suspend judgment about something (whether it be the truth of some statement or the existence of some thing). Someone who is agnostic about x does not make affirmative claims about the truth or existence of x, and in fact is specifically trying to avoid such claims. For an agnostic, lack of belief says nothing about the truth or falsity of a statement or the existence/non-existence of a thing. An agnostic lacks belief precisely because they do not want to assert either x or not-x.

Ultimately, I think some of the people answering your question are getting caught up in the “if I don’t believe in it, it doesn’t exist” part of the target statement and missing the “whether I see something or not” part of it. The opening clause makes the claim particularly ambitious since it dismisses the relevance of observation in belief formation.

@gorillapaws It’s one thing to argue that the necessity of a belief confirms existence, but it’s quite another thing to argue that belief (or the lack thereof) can determine existence (or non-existence). Where does Descartes ever argue for the latter?

@SavoirFaire “Where does Descartes ever argue for the latter?”

What about his ontological argument for the existence of God?

The ontological argument claims to prove the existence of God as a logical necessity. The existence supposedly follows from the definition of God as a perfect being. If the argument is valid, you have no choice but to believe in God’s existence.

@LostInParadise: Translated, what you say supports or denies the affirmation in the question?

It denies, because you no choice but to believe, if you accept the validity of the argument. It is interesting because the argument does not follow from observation but from logic. The ontological theory of God has a long history. I don’t know if it is still openly debated. Personally; I go along with Kant’s argument that existence is not a property of an object, and so can’t follow from the definition of the object.

An interesting twist on the original proposition is to ask if something exists simply because I believe in it. What would James say about the placebo effect? Are sugar pills a panacea because of all the documented cases where they alleviated disease symptoms?

@gorillapaws “What about his ontological argument for the existence of God?”

Descartes’ ontological argument says that once we examine our idea of God, we will see that He must exist. The necessity of our belief confirms God’s existence. But the argument doesn’t say that our idea of God causes God to exist (nor that ceasing to believe in God would cause God to cease existing, even if He were standing right before our eyes when we stopped believing). Our belief does not determine God’s existence.

If I might be allowed to get deep into the weeds here, Descartes’ argument for the existence of God is supposed to operate on the same principles as the cogito. The cogito says that once you examine your idea of thinking, you will realize that there is no way to doubt that you are both thinking and existing. Descartes’ version of the ontological argument for God, meanwhile, says that once you examine your idea of God, you will realize that there is no way to doubt that that something answering to that description must exist.

But while both of these conclusions are initially approached in the guise of a deductive argument, that aspect is ultimately discarded. In the end, the existence of the self and the existence of God are delivered by what Descartes calls the “natural light of reason.” Note that the natural light of reason is not the same as logic. It can’t be since the possibility of making an error in between steps would introduce room for doubt. Instead, the natural light of reason is some kind of pure intellectual capacity that precedes more structured forms of reasoning, and this capacity directly apprehends that “I think” cannot be separated from “I exist.”

This is clearer in the French version of the text, where the final formulation of the argument is “je pense, je suis” (“I think; I am”). The first formulation—“je pense, donc je suis” or “cogito, ergo sum”—suggests that there is a deductive move being made here, but Descartes realizes that he needs to shed that aspect of the argument (even if it is a helpful way of introducing it). As noted above, this is because the evil demon, or even just plain old incompetence, could introduce error if we had to conclude “I exist” from “I think.” The only way to avoid this is to have them be aspects of the same thought.

If this reading of the argument seems utterly unfamiliar, it’s not your fault. While it is more or less uncontroversial among Descartes scholars, my experience is that only oversimplified versions tend to be presented in any context below a graduate-level history of philosophy class. It’s a classic case of not trusting the audience, which is a tendency among academics that I rather dislike.

I have seen Descartes’ argument paraphrased (I think this was Sartre’s interpretation) as I doubt, therefore I am. It was not so much the ability to think so much as the ability to question that necessitated his existence. The one thing that he could not doubt was that he was engaged in the act of doubting.

@SavoirFaire ”The necessity of our belief confirms God’s existence. But the argument doesn’t say that our idea of God causes God to exist”

Well said. Thanks for taking the time to “get in the weeds.” You have a knack for writing clearly on complicated ideas. I, for one, am grateful for your efforts.

But in order for one’s belief to “confirm” God’ existence, one has to first be taught to believe that God actually DOES exist, if only in our minds.

In that regard, if one quits believing, then he does cease to exist in the mind of the person who quit believing….because his existence was all in their mind to begin with.

But that only applies to spiritual beliefs, which can be anything anybody wants them to be. It doesn’t apply to the physical world.

@Dutchess_III , You have to see how clever the ontological argument is. Suppose someone has no concept of God. I say, let’s define God as that which has the highest possible level of perfection, capable, for example, of creating universes. There is no problem, because we can make up any definition we want. Now we ask, does God exist? If God does not exist, God would be less than perfect, violating the definition. So, by definition, God must exist. Now we just have to work out what it means to have the highest possible level of perfection.

“Oh what a tangled web we weave when first we practice to deceive….” Moody Blues. But, just guessing from the words and sound of it, they stolz it from Shakespeare.

If something is actually real, and actually perfect, there can’t be any arguments against it that can’t be answered, right? But if you come up with a question that can’t be answered then you have to adjust your belief to somehow compensate for it.

@Dutchess_III Taht quote is actually from Robert Burns, the poet.

Ah. Thanks @janbb.

@Dutchess_III Oops. I was too quick to be a know it all. Just checked and apparently it was Sir Walter Scott in 1808. Sorry bout that.