being is the subject-matter of ontology. According to long tradition, there are kinds of being and modes of being. The kinds of being may be subdivided in various ways: for instance, into unviersals and particulars and into concrete beings and abstract beings. Another term for 'being' in this sense is 'entity' or 'thing'. In a second sense, being is what all real entities possess - in other words, existence. Being in this second sense has various modes. Thus the being of concrete physical objects is spatio-temporal while that of abstract mathematical entities like numbers is eternal and non-spatial. Again, the being of some entities (for instance, qualities) is logically dependent upon that of others, whereas the being of substances is logically independent.
Ted Honderich: The Oxford Companion to Philosophy, Oxford University Press 1995

Sein (gr. einai, lat. esse), in der Ontologie bezeichnet das Sein (gr. ousia) im Unterschied vom Seienden, dem Dasein und Sosein einzelner Dinge das Existieren von Dingen überhaupt, das "Sein des Seienden", das "Identische in der Mannigfaltigkeit des Seienden..... (N. Hartmann, Zur Grundlegung der Ontologie, 1935, S. 41).
J. Hoffmeister: Wörterbuch der Philosophischen Begriffe, Meiner Verlag, 1955

The traditional starting point for the question of being....are the fragments of Parmenides. There being is distinguished from non-being in terms of the distinction between the way of truth and the way of opinion. There can be no transition from non-being to being, no change or motion; being is all that can be known, and is one. Plato both softened and intensified Parmenides' distinction between being and non-being. The latter is no longer the absolute opposite to being, but participates in being to varying degrees; being at once informs the ideas as well as forming a higher idea in itself. Aristotle in the Metaphysics however emphasizes the participation of discrete beings in Being in general, establishing a repertoire of ways in which Being may be spoken of beings. He makes a crucial distinction between energeia and dynamis, which later evolved first into that of esse and essentia and then into that of existence and actuality.

The Scholastics agreed in distinguishing between being as existence, being as actuality and being as such: being as existence, or esse designated the existence of an essence, as in the 'being' of mankind; being as essentia designated the individual here and now actuality of, say, this woman or man; while being as such, that being whose essence is existence and actuality, can only be said of God.
For the moderns...esse now designates possibility, or that which is without contradiction, while essentia are those phenomena which are perceived to exist, while being as such is now taken to refer to the privileged being in itself, whether this is described in terms of God or causa sui, the subject, or the being-in-itself of the new, post-Cartesian science of ontology.
Howard Caygill: A Kant Dictionary, Blackwell 1995

BEING-IN-ITSELF: An existent can not be stripped of its being; being is the ever present foundation of the existent; it is everywhere in it and nowhere....
Being can not be causa sui in the manner of consciousness. Being is itself. This means that it is neither passivity nor activity.... Being is equally beyond negation as beyond affirmation.
But if being is in itself, this means that it does not refer to itself as self-consciousness does. It is this self. It is itself so completely that the perpetual reflection which constitutes the self is dissolved in an identity. That is why being is at bottom beyond the self... This can be better expressed by saying that being is what it is.
Being-in-itself has no within which is opposed to a without.... The in-itself has nothing secret... In a sense we can designate it as a synthesis. But it is the most indissoluble of all: the synthesis of itself with itself.
Being is. Being is in-itself. Being is what it is. These are the three characteristics which the preliminary examination of the phenomenon of being allows us to assign to the being of phenomena.
Jean-Paul Sartre: Being and Nothingness, Gramercy Books 1994, p. lxii ff.

Das "Sein" ist der "allgemeinste" Begriff: τὸ ὄν ἐστι καθόλου μάλιστα πάντων (Aristotles, Met. B4, 1001 a21). Ilud quod primo cadit sub apprehensione, est ens, cuius intellectus includitur in omnibus, quaecumque quis apprehendit. "Ein Verständnis des Seins ist je schon mit inbegriffen in allem, was einer am Seienden erfasst." (Thomas v. A., S. th. II qu. 94 a2). Aber die "Allgemeinheit" von "Sein" ist nicht die der Gattung. "Sein" umgrenzt nicht die oberste Region des Seienden, sofern dieses nach Gattung und Art begrifflich artikuliert ist: οὔτε τὸ ὄν γένος (Aristoteles, Met. B 3, 998 b22). Die "Allgemeinheit" des Seins "übersteigt" alle gattungsmässige Allgemeinheit. "Sein" ist nach der Bezeichnung der mittelalterlichen Ontologie ein "transcendens".
Martin Heidegger: Sein und Zeit, Max Niemeyer Verlag Tübingen, 1986, p. 3

The function and role of the brain as a medium of manifestation of the Individual Mind (Exonoesis) can be explained as follows:
Exonoesis is dependent on the physical brain and on the body as a whole. The brain can thus be called the carrier of Exonoesis, its medium of expression, its memory and material repository, useful for language and communication. Exonoesis is individually structured, according to the level of perfection reached within the biological evolution of the species. The more advanced the evolution of the brain is, the more highly sophisticated and subtle noetic operations can be executed by Exonoesis. Therefore it is not, as commonly held, that our mind actually developed together with the biological evolution, but only the receptivity and availability of the physical brain has evolved, while the faculties and potentialities of Exonoesis were adaptively actualized in the progressive development of the brain's state.

The brain is therefore just a medium of expression for thinking. Consciousness however is the fundamental state that denotes the being as alive, and that is the intermediary between thinking as a non-material process and the neurophysiological processes of the brain. Consciousness is the link between thinking and the brain, between the mind and the body. (see Essay Mind and Brain Relationship)

Against reducing mind processes to brain processes, see Essay Against the Theses of Biological Reductionism:

There is enough proof that thought can transcend the narrow set of functions of the brain. The reason that we can have thoughts going beyond the biological restraints of our brain, proves the immateriality and independence of our mind from matter.

See also my thesis that mind cannot have emerged from the brain, because it is something completely different from the underlying neuro-chemical processes. (see Essay Mind and Systems Theory)