Many philosophers have wrestled with the idea of consciousness, empiricism and the idea that consciousness is all we have with regard to reality. Schopenhauer's famous line, "The world is my idea' means that experience is always from the perspective of a perceiving consciousness. In other words, we represent the world to ourselves rather than have immediate access to the underlying nature of reality. We are, of course, talking about metaphysics, a difficult topic to be sure and this question of what the ultimate nature of reality is, is one that philosophers have not be able to solve. Kant believed there was the world as we experience and the underlying reality of the thing-in-itself. In our perceptual reality we impose categories of time, space and causation--we aren't passive recipients of sensory information. However, at the underlying level, the world as will, as Schopenhauer called it, these categories do not apply. The underlying reality is all that really exists.
Empiricist philosophers like David Hume distinguished two sorts of knowledge: relations of ideas and matters of fact. Kant recognized a third type of knowledge, what he called the synthetic or a priori. He uses synthetic as opposed to analytic--if a statement is not true by definition then it is synthetic (a priori is a Latin phrase which refers to any knowledge known to be true independent of experience). An example of a synthetic a priori statement would include most of mathematics, for instance, 1+1=2. An analytic a priori statement would be: all cats are animals--this is a judgment that does not give us new knowledge about the world. The third, empirical judgment (called a posteriori), would be something like all readers wear glasses. This last statement requires observation in order to falsify or verify it.
As mentioned in a previous post, one extreme of perceptual reality is solipsism: the view that all exists is one's mind. If the only things which one can experience are one's own ideas, not only does this lead to the view that there are no physical objects, but that there are no people! It was Sartre, not Descartes as I mentioned previously, who said that in almost every action we all of us suggest that we believe that there are minds other than our own. An example is the social emotion of shame. If one is could doing something they would not like others to see them doing they will most likely feel shame. Solipsism is invalidated by this feeling since to a solipsist, the concept of shame is meaningless as his mind is the only mind in existence and there would be no one to blame him.
Some of what were are talking about here on this DB is called phenomenalism; a theory of perception based upon the idea that we only ever have direct access to sense experience, never to the external world. There is also casual realism which assumes that the causes of our sense experience are physical objects in the external world. A problem with causal realism is that is doesn't take enough into account what it is actually like to see something, the qualitative aspect of sight. In other words, it reduces the experience to a form of information gathering. Also, causal realism makes what is known as a metaphysical assumption--that there is a real world out there that exits independently of people perceiving it.
Sartre's 'existence precedes essence' (thank you luctruc for correcting me) means that there is no pre-existing blueprint for humanity to which we must conform: e.g. human beings choose what they become. Sartre drew attention to the differences in conscious and non-conscious beings. The former he called "being for-itself' and the latter he called 'being in-itself.' Being for-itself is the kind of existence experienced by human beings, while in contrast, being in-itself is the being of non-conscious things, for example, a rock. Sartre describes human consciousness as a gap at the heart of our being. Consciousness is always conscious of something. It is never simply itself. Consciousness is what allows one to project oneself into the future and to reassess the past.
Now here is the truly fascinating part of Sartre ideas of consciousness. He describes the phenomenon of the ability of the human consciousness to see things as missing--he called this transcendence of consciousness. Here is an example: you make a appointment with someone for lunch. You arrive 30 minutes late and the person is not there. You are aware of him as a lack, an absence because you expected to see this person. Now, this is quite different from the absence of say, chess fiend and luctruc because you did not expect to meet them (one was playing chess and the other reading and listening to Bach). Only the person you arranged to meet would be felt as a genuine lack. This is Sartre's "nothingness." These two concepts, being and nothingness were what made Sartre's philosophy so original.
One final thought regarding Sartre (if you've read this far and I don't blame you if you scrolled down to a more interesting post and just went back to your game). He believed in free will. To him consciousness is empty in that it doesn't determine what we choose.
