The oldest writings that question 'what is consciousness' are the ancient Indian writings called Vedas (from 1500 BC). The question rises again with the Greek philosophers, and last century's the question regularly pops up in relation to the mind-body discussion in which the brain is often regarded as a 'material cause' of consciousness. For example, in the 19th century Thomas Henry Huxley argues that our consciousness is an "epiphenomenon", that is an additional effect of the brain function, a kind of steam cloud coming out.
The question of consciousness came back to attention at the beginning of the 20th century with the emergence of quantum physics. At the level of the smallest particles, miraculous things happen. For example photons and electrons behave differently when they are observed. An electron is actually 'everywhere and nowhere at once' . It only ends up in a place in our time-space dimension when it is observed. Scientists avoid the word consciousness and say that a system on nano-scale behaves differently when 'information is extracted' from it.
But by whom? I guess by a conscious entity, otherwise information can not be perceived. This is also apparent from recent experiments in which data about the movement of electrons were stored on a computer. This does not influence the events (the way electrons travel) until these (which way) data are viewed by someone. Then the behavior of the electrons are changing, even back in time! That is called retrocausality. Changing the cause of something back in time.
In a personal sense consciousness is something you can lose. Neurologists discovered that - as we lose consciousness - there is less connection (communication) between the different brain centers. But sometimes it is possible to communicate with someone in coma, and even people who are temporarily clinical dead appear to have consciousness as the neurologist Pim van Lommel demonstrated.
In scientific articles, the term consciousness is used in many ways without giving a clear definition of what exactly is meant by it. That is quite confusing. In the literary appendix of the Times (1992), the philosopher Jerry Fodor complains: "No one has any idea about how matter or physiological systems can be aware. Why? Do neurologists still not know enough? Or are we looking for clues in the wrong place? Researchers focus on brain processes and neural networks. But what if we instead would focus on what is not there?
The question 'what is consciousness' also depends on whether we use consciousness in a psychological (personal) context or in a scientific such as the consciousness of cells.
In the latter case, a definition of consciousness could be: "knowing something about (the state of) something else and reacting to it" . Then microbes and plants also have consciousness because they react and change according to circumstances. There are trees that produce perfumes (pheromones) during an attack by insects to warn other trees. So trees are somehow aware, do they have conciousness?
In biology more and more research is showing how this happens at a cellular level and how consciousness plays its role. As it seems, the awareness of living systems is a dynamic process of information exchange, making use of knowledge that exists 'somewhere', distributed over many systems, intertwined not bound to place or time.
More and more scientists think that consciousness plays a cardinal role in physical processes. Donald Hoffman (professor of cognitive sciences) writes:
"It is a universe of conciousness, or concious agents having experiences. Conciousness is the fundamental nature of reality".