‘Mind’ is a traditional concept which may have arisen from the idea of pneuma (breath of life, psyché, spiritus) that makes the body alive and leaves the body in dying. The belief of the immortality of the soul has been related to this idea. The contemporary philosophic concept of mind mainly dates from Descartes, who used the terms mind and soul synonymously:
“I am then in a strict sense only a thing that thinks: that is, I am a mind, or soul, or intellect, or reason...” (res cogitans, id est mens, sive animus, sive intellectus, sive ratio) 
Descartes regards the mind as something individable:
“... that a great difference exists between mind and body insofar as the body is always dividable by its nature, the mind, by contrast, is quite individable. […] It is also not correct to refer to the abilities of volition, cognition, etc. as its parts, since it is one and the same mind who wants, feels, or recognizes. ” 
In contemporary scientific debate, the human mind is rarely seen in this way (as the one, potentially immortal counterpart of the body). The mental is rather construed as a property of a class of states or events that are conscious and, at least in naturalist approaches, take place in the brain. That is, a crucial feature of mental states like perceptions, thoughts, feelings, desires, etc. is, that they are conscious to us. Off course, many things in the brain happen unconsciously, and although we are not aware of them, they can influence our conscious experience and our behavior. Let us, for simplicity’s sake, preliminary regard all unconscious brain states and -processes as physical (below it will become clear that some of them possess a certain feature of the mental).
The mind is no longer regarded a supernatural entity by almost all philosophers. However, the current scientific concept of the mind is similar to Descartes’ view in the assumption the mental is one property, not several different properties. Indeed, there is a differentiation between phenomena like thoughts, perceptions, feelings, memories, desires, but all these phenomena are called ‘mental’ since this one property is attributed to all of them. The mental is regarded as a natural kind.
In contrast to that, the position I will discuss on this website is: The mental is not a natural kind, but rather a combination of two phenomena of different origin. I think the belief of the mental as a natural kind has been the main obstacle in solving some problems in the philosophy of mind, among them the relation between states in the brain and mental states and the issue of how the human mind could evolve in a purely material world.
For further discussion, we need a preliminary definition of the term ‘mental’: Is there a further crucial common feature of such different mental phenomena like thoughts, perceptions, emotions, desires, or memories which makes them appearing as one kind of phenomena – besides the fact that they are conscious to us? Yes, there is: they all are ‘intentional’, that is, they are ‘about’ something, they have a content, they refer to something outside themselves. The term intentionality dates from medieval scholastic philosophy and was ‘rediscovered’ by Franz Brentano in the 19th century. He wrote;
“Every mental phenomenon includes something as object within itself, although they do not all do so in the same way. In presentation something is presented, in judgment something is affirmed or denied, in love loved, in hate hated, in desire desired and so on. This intentional in-existence is characteristic exclusively of mental phenomena. No physical phenomenon exhibits anything like it. We could, therefore, define mental phenomena by saying that they are those phenomena which contain an object intentionally within themselves.” 
One may argue that there are some mental states referring to themselves only, like pain, melancholy, happiness without reason. or the state of serenity and non-directed awareness in meditation. I think also these mental states are about something different from themselves, even if their content is of more general nature: They are about being alive. Just when we experience pain, or sadness, or happiness, or the awareness in meditation, we feel and realize that we are alive. In the movie ‘Linie 1’ (about the Berlin metro), an old man says: If you wake up in the morning, and nothing hurts, then you are dead.
However that may be, the issue of non-intentional consciousness is irrelevant in our context. For our purpose, it is only important that intentionality is a sufficient criterion of the mental (even if it my not be a necessary one), because I don’t want to defend the position that physical and mental phenomena are disparate, but the position that they have some important common features. Thus I simply follow Brentano by saying: All what is mental is also intentional. However, I do not agree with Brentano in another point: In the next section I will propose: there are physical entities that are, at least in a sense, intentional.