If something exist mentally & physically, in both forms it is understandable that we have a Idea of it in mind as well as in reality or physically but , what about things which exists in the form of Idea only? A person exist means is it he/ she alive? If that person dead he is not exist? If we consider living beings life as the prove of their existence then what about a stone who don't have life isn't that exist? If something which is doesn't exist in our ideas & Al's in our peripheral world we never seen that or have any idea of it can we called something like that is exist? Idea's of our mind doesn't exist in real?
You have answered your own question without releasing it. Ideas of the mind exist as ideas of the mind. If I imagine a giant pink unicorn, then it exists in an abstract sense. It does not exist in the same way that a horse in a neighbouring field exists. So the word 'exist' has several meanings, and much confusion arises where people fail to maintain the distinctions between them.
Here is an answer based on Heideggerian phenomenology. Phenomenology proceeds from what one is most certain of: one's own experience of reality. Heidegger extends Kant's phenomenology to define subjective time. Whereas Kant described a self acting over a backdrop of intuitions of time and space Heidegger brings the instantiation of time right into the constitution of the self: "time ... forms the essential structure of subjectivity", i.e. quoting from Kant and the Problem of Metaphysics, §34. Taft translation. (Alternate Churchill translation here.), original here: p. 189 Comments added: *
Time is only pure intuition to the extent that it prepares the look of succession from out of itself ... This pure intuition activates itself with the intuited which was formed in it, i.e., which was formed without the aid of experience. According to its essence, time is pure affection [as in something which affects something]* of itself. ...
As pure self-affection, time is not an acting [external]* affection that strikes a self which is at hand. Instead, as pure it forms the essence of something like self-activating. However if it belongs to the essence of the finite subject to be able to be activated as a self, then time as pure self-activation forms the essential structure of subjectivity.
This means when there are thoughts happening in a sequence, that means time is involved. This is the minimum that can be said on the matter without bringing in extra complexities such as 'intuitions'. However unsatisfactory it may seem, this is the simple, functional definition of the self, determined from what it is: a temporal agent.
Once selves are up and thinking and wondering how long a day lasts they can invent the caesium standard of objective time, but this is man-made and only applies to regular spacetime not singular regions. Suffice it to say the completed physics of time is still awaited.
Returning to the phenomenological self. With eyes open, space is experienced. Heidegger calls this space 'the clearing'. Its existence is somewhat mysterious because it is literally the space where 'things' can exist, but it itself is not those things.
The existence of the self (as experiencer) is of a higher order than the existence of things. However, one can see oneself as an objective thing, so one has subjective existence and objective existence.
Objective existence was revolutionarily defined by Kant as a fusion of material encounter and cognition. This can include the material encounter of a thought or a dream too. Kant's formulation is made in the Critique of Pure Reason A598/B626, original here:
... the determination of a conception is a predicate, which adds to and enlarges the conception. It must not, therefore, be contained in the conception.
Being is evidently not a real predicate, that is, a conception of something which is added to the conception of some other thing. It is merely the positing of a thing, or of certain determinations in it. Logically, it is merely the copula of a judgement.
The copula of a thing is its concept (e.g. coffee) plus its predicates (hot, strong), but existence is not a predicate. Existence occurs with the judgement. How existence is not a predicate is clarified at the end of the quoted paragraph:
in reckoning my wealth there may be said to be more in a hundred real dollars than in a hundred possible dollars—that is, in the mere conception of them. For the real object—the dollars—is not analytically contained in my conception, but forms a synthetical addition to my conception (which is merely a determination of my mental state), although this objective reality—this existence—apart from my conceptions, does not in the least degree increase the aforesaid hundred dollars.
Perceived existence does not change the hundred dollars.
So we can now have things existing in the clearing and are in a position to answer the OP's questions.
- what about things which exists in the form of Idea only?
There may be a complex [predicate] thought [concept] experienced [judged], so no problem with that existing.
- A person exist means is it he/ she alive? If that person dead he is not exist?
A living person's subjective existence is of a higher order than the objective existence of say, a chair, because the living person is making the judgement that brings existence to the chair. The higher, subjective existence of the person is experienced (at least while awake), and while features of this existence can be described (e.g. thrownness, care, carefulness in its predicament) it is not the same as the existence of a chair, or for that matter another person's body (objective existence), whether alive or dead.
- then what about a stone who don't have life isn't that exist?
If a person perceives it, makes a judgement, it exists.
- If something which is, doesn't exist in our ideas ... can we called something like that is exist?
It can exist in the set of things you have never seen or thought about, but that is stretching the point.
- Idea's of our mind doesn't exist in real?
Thoughts are real things.