How can we prove something exists?

Question

As someone who likes a lengthy argument with many different points put across and many points shot down in the process, I was posed with this question which has made me struggle so far. My friend said to me: "Prove something exists." I have managed to prove I exist however I cannot prove anything else exists. His argument went something along the lines of the theory that our mind just sends us inputs, whether they be visual or pain or audio or even psychological, he thinks that we could just be a mind located somewhere else.

My points against this have all been shot down, they go as follows:

• Dogs see 2 base colours, blue and yellow. They cannot see a red ball in green grass but can easily spot a blue ball in the grass. We see three and therefore we can see things that they cannot. Mantis shrimps see 16 base colours, and they can spot things out much more easily than we can. That means the colours they see exist and our minds cannot comprehend them, meaning that our mind can not have thought of them. However, he just said "what if your mind is just telling you how the shrimp behaves to make you think that it can see more colours than we can?" Which I had no response to.
• I then said that we need food to restore our energy and that energy can't just come out of nowhere so there must be something giving us the energy, which is the food. The food therefore exists, however he just said "What if your mind is just telling you to feel the effects of fatigue or tiredness?" which again I had no response to.
• My third and latest full argument was that mirrors show us things that previously we could not see. If you were to place a button on your back that electrocutes you, you would not know where it was until you saw it in a mirror, which must mean you exist because the mirror can see something that you have previously established does create an effect on you. You can use the mirror to see something that you previously could not see. That means the image in the mirror must exist, however he disregarded this by saying "what if your mind is telling you you can see it in the mirror and is creating that image to fool you into thinking it's there."

My next thoughts are those that there three options when the body dies:

1. The mind leaves the body, but this would mean that people who have a near death experience or are clinically dead (Such as Fabrice Muamba was for 78 minutes.) do not have a mind when they come back to life.
2. The mind dies when the body does, but this would have the same effect.
3. The mind stays with the body. This is the only logical explanation, so what would this mean?

Answer 1

A question like, How can I prove something exists? must be placed in a context. Who is asking, and what will they accept as proof?

In an ordinary everyday sort of way, it might be answered by saying that something is the object of our senses: I can prove this apple exists because I can see it, touch it, smell it, taste it. In less simple examples, one might use indirect evidence - I can't prove that Australia exists by sensing it, since it is a long way away from me, but I could point to photographs, books about it, people who have been there, beer that has been brewed there, etc.

Now if the questioner responds, But that's only evidence, not incontrovertible proof, he is upping the measure of what we ordinarily count as proof and playing a kind of game of radical skepticism. There have been several kinds of radical skepticism - one can be skeptical about the existence of other minds, which leads to solipsism; one can be skeptical about the past and believe that the entire universe came into existence a few seconds ago with all my memories preformed; one can follow Descartes and try to be skeptical about everything. Ultimately there is no way to prove such a skepticism to be mistaken, one can only say that it is pointless or unhelpful.

Incidentally, you say that you can prove that you exist, but are you sure? Descartes' cogito was famously criticised by Hume who said that he cannot discover in his own experience what "I" refers to. If you are going down the route of radical skepticism you cannot help yourself to concepts like "I" without proof, which will lead to a regress.

Answer 2

One of the philosophical attempts to resolve this issue is found in the work of Immanuel Kant, who proposed a method of Transcendental Argument. His idea was that we should first agree on a protocol of accepted proofs or principles of reasoning or experience, then following on from this to extract from it an ontological commitment to the mathematical or metaphysical structure necessary for such proofs or principles to be accepted. The conclusion will then end up either supporting the existence proof or result in a dispute about the original protocol.

Your opponent is keen to press the line that your brain might just be telling you these things. But presumably in order to even propose this line, it must be reasonable to suppose that your brain exists. So something exists.

Easy, right? Just press this line home - what does your opponent accept about the world in their various skeptical scenarios, and what does that commit them to?

Answer 3

X exists if it can lead to certain experiences, and you argue that X exists by relating it to other experiences.

For instance, if I'm in the desert with some friends and I see water in the distance, I may wonder if the water really exists. This means I'm wondering if the water will lead to certain experiences, such as feelings of wetness or a quenching of my thirst should I drink it. To confirm it exists, I'd ask my friends if they too see water. Thus I confirm that my experience of water will lead to the experience of thirst quenching because it is consistent with the experience of hearing others say they see water.

So when your friend asks you to prove X exists, your friend is asking you to show that X can cause certain experiences. Once you've done that, you've won the argument.

If your friend rejects this by arguing that he could be a brain in a vat, then your friend is equivocating. He is using a different definition of "exists" than is warranted by the argument. Argument occurs in the context of experiences, it can't be used to justify or reject the ultimate "source" of experience.

Call your friend out on his equivocation and call it a day.

Answer 4

You're asked by your friend to

Prove something exists

The word 'prove' suggests that formal reasoning must be deployed; thus he is implicitly disallowing you to point to the jar of biscuits and say 'that exists'.

Turning then to Aristotles Organon his six books on logic) we see that he uses the word 'deduction' (sulligimos); so we ask - what, then is meant by a deduction? Aristotles reply is:

A deduction is speech, in which certain things having been supposed, something different from those supposed results from neccessity because of their being so (Prior Analytics I.2)

Also, each of the 'things supposed' is a premise (protasis); and 'what results of neccessity' is the conclusion (sumperasma).

It's how we get from the premises to the conclusion 'by neccessity' that is the notion of proof.

Thus a deduction, otherwise known as a valid argument, is to go from premises to the conclusion by way of a proof.

Thus, given no premises you have no way of proceeding: 'from nothing can only come nothing'!

Thus you must ask for premises; and thus the deduction will turn on the nature of premise.

Now, if the premise given is of concrete objects such as:

Chryssipus is a man

You can point to Chrysippus and say he existed at some point (where you use a tensed notion of existence).