<P> The 20th - century German philosopher Martin Heidegger criticized the ontological assumptions underpinning such a reductive model, and claimed that it was impossible to make sense of experience in these terms . This is because, according to Heidegger, the nature of our subjective experience and its qualities is impossible to understand in terms of Cartesian "substances" that bear "properties". Another way to put this is that the very concept of qualitative experience is incoherent in terms of--or is semantically incommensurable with the concept of--substances that bear properties . </P> <P> This problem of explaining introspective first - person aspects of mental states and consciousness in general in terms of third - person quantitative neuroscience is called the explanatory gap . There are several different views of the nature of this gap among contemporary philosophers of mind . David Chalmers and the early Frank Jackson interpret the gap as ontological in nature; that is, they maintain that qualia can never be explained by science because physicalism is false . There are two separate categories involved and one cannot be reduced to the other . An alternative view is taken by philosophers such as Thomas Nagel and Colin McGinn . According to them, the gap is epistemological in nature . For Nagel, science is not yet able to explain subjective experience because it has not yet arrived at the level or kind of knowledge that is required . We are not even able to formulate the problem coherently . For McGinn, on other hand, the problem is one of permanent and inherent biological limitations . We are not able to resolve the explanatory gap because the realm of subjective experiences is cognitively closed to us in the same manner that quantum physics is cognitively closed to elephants . Other philosophers liquidate the gap as purely a semantic problem . This semantic problem, of course, led to the famous "Qualia Question", which is: Does Red cause Redness? </P> <P> Intentionality is the capacity of mental states to be directed towards (about) or be in relation with something in the external world . This property of mental states entails that they have contents and semantic referents and can therefore be assigned truth values . When one tries to reduce these states to natural processes there arises a problem: natural processes are not true or false, they simply happen . It would not make any sense to say that a natural process is true or false . But mental ideas or judgments are true or false, so how then can mental states (ideas or judgments) be natural processes? The possibility of assigning semantic value to ideas must mean that such ideas are about facts . Thus, for example, the idea that Herodotus was a historian refers to Herodotus and to the fact that he was a historian . If the fact is true, then the idea is true; otherwise, it is false . But where does this relation come from? In the brain, there are only electrochemical processes and these seem not to have anything to do with Herodotus . </P> <P> Philosophy of perception is concerned with the nature of perceptual experience and the status of perceptual objects, in particular how perceptual experience relates to appearances and beliefs about the world . The main contemporary views within philosophy of perception include naive realism, enactivism and representational views . </P>

Any substance that acts on the mind is referred to as a