What is the mind? Of course, "mind" is just a word, and we can mean anything we want by it. But if we examine the way we use the word, and think about the kinds of things we consider more mindful than others, I would argue that the idea of choice is the most important. We consider things to be more or less mindful to the extent that they appear to be making choices. To make a choice means to distinguish, and to create a difference. In this basic sense the mind is about information. Its essential function is to process bits into other bits. This position has two elements:
The idea that the mind's activities are best viewed as information processing, as computation, has become predominant in our sciences over the last 40 years. People do not doubt that minds have physical, material form, of course, either as brains or perhaps as computer hardware. But, as is particularly obvious in the latter case, the hardware is often unimportant. Is is how the information flows which matters.
I like to bring this idea down to our basest intuition. What things are more mindlike and less mindlike? A thermostat is slightly mindlike. It converts a gross physical quantity, the air temperature of your home, to a small deviation in a piece of metal, which tips a small lump of mercury which in turn triggers a fire in your furnace. Large physical events are reduced and processed as small ones, the physical is reduced to mere distinctions and processed as information. The sensors and effectors of our brains are essentially similar. Relatively powerful physical forces impinge on us, and our sensors convert them to tiny differences in nerve firings. These filter and are further processed until signals are sent to our muscles and there amplified into gross changes in our limbs and other large physical things. At all stages it is all physical, but inside our heads there are only small physical quantities that are easily altered and diverted as they interact with each other. This is what we mean by information processing. Information is not non-physical. It is a way of thinking about what is happening that is sometime much more revealing and useful than its physical properties.
Or so is one view, the view that takes a material physical reality as primary. The informational view of mind is just as compatible with alternative philosophical orientations. The one I most appreciate is that which takes the individual mind and its exchanging of information with the world as the primary and base activity. This is the so-called "buttons and lights" model, in which the mind is isolated behind an interface of output bits (buttons) and input bits (lights). In this view, the idea of the physical world is created by the mind so as to explain the pattern of input bits and how they respond to the output bits. This is a cartoon view, certainly, but a very clear one. There is no confusion about mind and body, material and ideal. There is just information, distinctions observed and differences made.
Implicit in the idea of choice, particularly as the essence of mindfulness, is some reason or purpose for making the choices. In fact it is difficult even to talk about choice without alluding to some purpose. One could say a rock "chooses" to do nothing, but only by suggesting that its purpose is to sit still. If a device generated decisions at random one would hesitate to say that it was "choosing." No, the whole idea of choice implies purpose, a reason for making the choice.
Purposiveness is at heart of mindfulness, and the heart of purposiveness is the varying of means to achieve fixed ends. William James in 1890 identified this as "the mark and criterion of mentality". He discussed an air bubble rising rising in water until trapped in an inverted jar, contrasting it with a frog, which may get trapped temporarily but keeps trying things until it finds a way around the jar. Varying means and fixed ends. In AI we call it generate and test. Or trial and error. Variation and selective survival. There are many names and many variations, but this idea is the essence of purpose, choice, and Mind.