What is the difference in concrete things between being and existing, or is there none?

Question

I am supposing that being and existence must correspond to principles in concrete things, else the notions of being and existence are false precisely because they are attributed to things. So what exactly is it in things (for example, a cat or dog) that being has but existence does not and vice versa? Note that I am not merely looking for the conceptual difference (obviously, they are different conceptually), but the real difference in things (i.e., in re) if such a difference exists... pun not intended.

Answer 1

The problem with which Russell and Meinong were dealing was the following kind of line-of-reasoning:

1. For a description d^r to be true of something A, A must exist.
2. Suppose A doesn't exist.
3. Then here d = "doesn't exist."
4. In order for it to be true, then, that A doesn't exist, A must exist in the first place.
5. Ergo, everything whatsoever has some sort of existence.

Russell mostly, in the end, resisted, while Meinong wholeheartedly embraced, (5), such that there was then a desire to identify a more robust use of words like "existence" and "being" and the like. What might've in an older time been a full-fledged theory of degrees-of-existence became just the minimal such postulate (of two such degrees). Now, you do specify that you are asking about how such a difference might apply to concrete objects, and here the prevailing approach to the matter is Zalta's, for whom the special nature of the abstract/concrete difference is what grounds our ability to speak truthfully about existing abstract objects. Mind you, the prevailing gloss of the very notion of existence, nowadays, is with very good reason the quantifier-theoretic one, i.e. to say, "X exists," is "actually" to say, "The number of things that are X is at least 1" (or "at least a positive nonzero number," if the existence of intrinsically fractional or even infinitesimal things is allowed for). (This due to the matter of Anselm's old, weird argument for God, with which Russell also dealt, chiefly by way of the "existence is not a predicate" consensus that emerged from Kant through Frege.)

So then we would look for two "modes" of this minimal positive quantification. In fact, as we just indicated, one might, for example, imagine partial existence, as in partial, i.e. fractional, existence, vs. full existence. Or there might be trivial-vs.-nontrivial, external-vs.-internal, possible-vs.-actual-vs.-necessary, etc. variations in the moment of quantification. Perhaps unfortunately for Meinong, then, it is less that his thesis is outright and entirely wrong, but that it is acceptable for a trifling logical reason, for the sake of a peculiar play in a peculiar, but generically allowable, language game (the quantifier game, that is).

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^rOr one might construct a similar thought process more on the basis of the concept of reference than of description.

Answer 2

Paraphrasing OP: what exactly is it in things (for example, a person) that being has but existence does not?

Heidegger distinguishes the 'subjective' existence of a person (the Being of Dasein) from the objective existence of things, which he calls "presence-at-hand". A person can be present-at-hand even to theirself, but their Being is what makes them alive so to speak.

The 'essence' ["Wesen"] of this entity [i.e. Dasein] lies in its "to be" [Zu-sein]. Its Being-what-it-is [Was-sein] (essentia) must, so far as we can speak of it at all, be conceived in terms of its Being (existentia). But here our ontological task is to show that when we choose to designate the Being of this entity as "existence" [Existenz], this term does not and cannot have the ontological signification of the traditional term "existentia"; ontologically, existentia is tantamount to Being-present-at-hand, a kind of Being which is essentially inappropriate to entities of Dasein's character. To avoid getting bewildered, we shall always use the Interpretative expression "presence-at-hand" for the term "existentia", while the term "existence", as a designation of Being, will be allotted solely to Dasein. GA 2, H. 42

For presence-at-hand Heidegger generally follows Kant's definition for the objective existence of things, as given in The Critique of Pure Reason :-

Through the actuality [i.e. objective existence] of a thing I certainly posit more than possibility, ... [While] possibility was merely a positing of a thing in relation to the understanding (to its empirical use), actuality is at the same time its connection with perception. B287

Being [objective existence, in this context] is obviously not a real predicate, i.e., a concept of something that could add to the concept of a thing. It is merely the positing of a thing or of certain determinations in themselves. In the logical use it is merely the copula of a judgment. B626

The copula is the connection of idea/possibility plus (evidenced by) sense perception data, thus constituting the thing's existence for the observer [Dasein].

Treating objective existence as 'knowledge', when knowledge corresponds to the facts this is truth. However establishing the facts is tricky since the effects a thing might reveal may be misleading as to what the facts of the matter are. Heidegger calls this the twofold concealment.

Concealment can be either a refusal or merely an obstructing. We are never really certain whether it is the one or the other. Concealment conceals and obstructs itself. ...

The essence of truth, i.e., unconcealment, is ruled throughout by a denial. This denial is, however, neither a defect nor a fault – as if truth were a pure unconcealment that has rid itself of everything concealed. If truth could accomplish this it would no longer be itself. Denial, by way of the twofold concealing, belongs to the essence of truth as unconcealment. (The Origin of the Work of Art (1935-1936), pages 30–31)

So we can't be certain of the truth of the existence of things. Hence in Contributions to Philosophy (1936–40) Heidegger writes:

Is there indeed truth? How? If truth were not, on what would stand even the mere possibility of the "why"? Does the why-question already confirm the fact that there is truth, that truth must be in some way or other? GA 65, §236. Truth

Returning to the Being of Dasein, concealment and unconcealment are considered to occur in The Clearing, which is where one's view of the world takes place.

In the midst of beings as a whole an open place comes to presence. There is a clearing. Thought from out of beings, it is more in being [Existenz] than is the being [entity]. This open center is, therefore, not surrounded by beings. Rather, this illuminating center itself encircles all beings – like the nothing that we scarcely know.

The being can only be, as a being, if it stands within, and stands out within, what is illuminated in this clearing. Only this clearing grants us human beings access to those beings that we ourselves are not and admittance to the being that we ourselves are. Thanks to this clearing, beings are unconcealed in certain and changing degrees. But even to be concealed is something the being can only do within the scope of the illuminated. (The Origin of the Work of Art, page 30)

So we come to one of Heidegger's most tangible elucidations of Being: "the clearing itself is being".

metaphysics recognises the clearing of being either solely as the view of what is present in “outward appearance” (ἰδἐα) or critically as what is seen in the perspect of categorial representation on the part of subjectivity. This means that the truth of being as the clearing itself remains concealed for metaphysics. [163] However, this concealment is not a defect of metaphysics but a treasure withheld from it yet held before it, the treasure of its own proper wealth. But the clearing itself is being. Within the destiny of being in metaphysics the clearing first affords a view by which what is present comes into touch with the human being, who is present to it, so that the human being himself can in apprehending (νοεἴν) first touch upon being (θιγεῑν, Aristotle, Metaphysics Θ, 10). This view first draws the perspect towards it. It abandons itself to such a perspect when apprehending has become a setting-forth-before-itself in the perceptio of the res cogitans taken as the subiectum of certudio. (GA9 Pathmarks, Letter on “Humanism” (1946) pages 252-253)

Being's relation to nothing is that we see nothing of Being. We see the phenomena; the unconcealed-concealed beings that constitute the world.

The nothing is the "not" of beings, and is thus being, experienced from the perspective of beings. (1949 preface to On the Essence of Ground, Pathmarks, p. 97)

Beyng is nothing "in itself" and nothing "for" a "subject." Only beingness can appear as this sort of an "in itself" and can do so only in the form of an effete φύσις [phusis/Nature], i.e., as ίδἐα, the καθ' αὐτό ["for itself"], something represented, an object. An extreme confinement in objectivity befalls all attempts to find "being" and its "determinations" (categories) in the manner of something objectively present. (GA 65, §270. The essence of beyng)