1. andrey_smirnov says:

Usually a question involves one of two options for working on it:

1) the possibility of reformulating it into a statement: is it true that the Sun is a yellow dwarf star? = �”The sun is a yellow dwarf”. Such a statement implies evaluating the truth in Boolean algebra as 0 or 1.�

2) the ability to answer in the form of a list of the most significant properties, which is equivalent to a dictionary definition. For example: “What is a parallelogram?” “a parallelogram is a quadrilateral whose opposite sides are pairwise equal and parallel.” The original question essentially boils down to the first case: finding properties that define an object.

Moreover, the logical notation of these statements must contain the existence quantifier∃. The symbol is read as “exists”. There is also the universal quantifier: чит is read as “all”, or “for all”.�

For example: ∃x P(x) – read as ” there exists x such that the property P(x) is true for it. This is the standard form for writing properties of any associated variable x in mathematical logic. A variable is called bound if it is affected by one of the above quantifiers.

Your question can be restated as follows:” list everything that exists”, or”name the essential properties of everything that exists”. Both options are equivalent. In the first case, a simple enumeration does not give us anything interesting.

Consider the second case. What is the existing one? In this case, the second option is suitable for us – to give a definition. For example: existing is something that exists. As can be observed, this formulation is tautological, and it does not add any new knowledge. In formula form, it looks like this: ∀x ∃x if ∃x – “for all x exists x if x exists”. All.

This was noticed by Immanuel Kant, and therefore he wrote that existence is not a property. This division exists in logic to this day – therefore, the existence and universality quantifiers are a separate type of logical symbols – these are not properties of P(x), but quantifiers.

The great German mathematician of the last century, David Hilbert, suggested that everything that does not lead to logical contradictions should be considered existing in mathematics. Therefore, we can afford to say that what exists in the world does not lead to logical contradictions with the laws of the universe.

We can recall the definition of existence given by the English analytical philosopher of language, mathematician and logician, Willard Quine – “to be is to be the value of a bound variable”. Because a bound variable, as I noted above, is bound by quantifiers of universality or existence. And if the variable really has some value (some object), then the statement about the existence of this object, written in the form of some set of characters, becomes meaningful. Otherwise, we are faced with linguistic nonsense.

2. andrey_kartashev says:

Сogito ergo sum. I think, so I exist.

I think it's hard to have doubts about the existence of yourself. But the existence of a world outside of your consciousness is already a more complex question. We are limited in our knowledge of the world by our senses. What we can't see, touch, hear, etc., simply doesn't exist for us.

But does this mean that there is everything that we can perceive with our senses? No, and again no. Our brain only processes and reproduces the information that comes to it, and we all know that it does not always cope with this task well. Using the same hallucinations as an example, we can understand that what subjectively exists for one person, objectively does not exist for the rest.

So we can be sure of the existence of the signals received and processed by our brains, and in the existence of precisely the final versions provided to our consciousness. About the true essence of things and its existence, we can only guess.