That we have a mental life of thoughts and perceptions is the most fundamental fact of human experience and the starting point for every other kind of inquiry. Colours and objects in our field of vision; intentions and beliefs; pains, memories, thoughts—the most radical forms of philosophical skepticism must take all these as properly basic even when denying everything else. 
There are, meanwhile, five properties of consciousness which are much-discussed in the Philosophy of Mind because it seems they cannot in principle be explained on a naturalistic ontology.  In other words, presupposing with the Naturalist that mindless particles organised in various ways by mindless forces is all that exists seems to leave us without the explanatory resources to account for our mental life.
The Argument from Consciousness begins here—with an a priori proof that these fundamental properties of consciousness are in fact insusceptible of reduction to the physical. It then draws out the logical entailments: For if the mind cannot possibly be reduced to the brain then mind and brain are not identical. Naturalism is falsified and some form of substance dualism is implicated. And given the existence of nonphysical mental substances established by the argument, theism is an inference to the best explanation for them.
In this five-part series of posts I will present each of these properties in turn and then argue that it is credibly probable that agents with these mental properties will exist if there is a God but incredibly improbable that they would exist if there is not. The existence of conscious agents with these five mental properties therefore provides evidence that there is a God who created them.
The hiss of car tyres on a wet road; the smell of jasmine or the taste of avocado; a flash of sunlight on a stormy lake. All these things have a raw qualitative “feel” that is as immediate and undeniable as it is indescribable. Philosophers call these subjective tinctures of sense perception qualia; and in his influential paper What Is It Like to Be a Bat?  the eminent philosopher of mind Thomas Nagel argues that they present an insurmountable conceptual challenge to naturalism.
Nagel begins by noting that if an organism is conscious at all then, “there is something it is like to be that organism.” To complete a naturalistic account of mind, this subjective savour of selfhood must be reducible to an objective brain state. The problem is that the reductive step by which a physical theory is arrived at translates what is private and subjective into what is public and objective—a point to which we shall return. Qualia, meanwhile, just are the private and subjective experiences of sense perception. And since quaila are also facts about the world it follows that there are facts about the world that naturalism cannot possibly explain.
To help us understand this point and its implications Nagel invites us to consider what it is like to be a bat. “Sonar,” he notes, “though a form of perception, is wholly unlike any sense that we possess and there is no reason to suppose that the subjective experience of a bat is like anything we can experience.” It will not do here, says Nagel, to imagine that you have webbed arms that enable you to fly around at dusk catching insects in your mouth; or that you perceive the world by means of high frequency sound signals; or that you spend the day hanging upside down by your feet in an attic—all this only tells you what it would be like for you to behave as a bat behaves and that is not the question. “I want to know,” Nagel writes, “what it is like for a bat to be a bat.”
How, then, can this be known? The answer is that it cannot because the task is impossible by tautology: Bat qualia can no more be instantiated in nonbat consciousness than triangularity can be instantiated in a circle. Limited to the resources of the human mind, the extrapolation to bat experience is incompleteable. And critically, the problem is not confined to such exotic cases. In contemplating bats, says Nagel, we are in the same position of an intelligent bat contemplating us. The structure of their minds make it impossible for them to succeed; and nor could they plausibly deny that there are qualia of human experience. We know what it is like to be us; know, that is, the ineffable but highly specific subjective savour of personhood from moment to moment. Nagel concludes that qualia are trapped within a particular point of view and can never survive transference to a physical theory open to multiple points of view.
This is the first property of consciousness that is insusceptible of reduction to the physical.
 Philosophical idealism takes a skeptical view of the external world and holds that reality is fundamentally mental; solipsism holds that only one’s own mind can really be said to exist. Descartes famously held that we can coherently doubt everything except the fact that we doubt—cogito ergo sum.
A belief is properly basic if it cannot be derived from other beliefs but must be accepted if beliefs of any kind are going to be possible. Other examples include the reality of the external world, the deliverances of rational intuition, and the existence of other minds.
 The five properties in the order they will be discussed in this series are: Qualia, Intentionality, Privileged Access, Nonphysicality and Free Will.
 Nagel’s fascinating essay is only 16 pages. You can read it here. See also his book Mind and Cosmos: Why the Materialist Neo-Darwinian Conception of Nature Is Almost Certainly False.