Like most early modern philosophers, Descartes believes that we do not perceive the external world directly. One of the main reasons is the possibility of illusions and hallucinations. Now why is that? The possibility of illusions and hallucinations requires us to single out pure sensory experiences without any interpretation or what we now call cognitive processing, as the entities between us and the world that we do perceive directly. We cannot be mistaken, we cannot possibly err in having the sensory experiences that we in fact have. We are infallible in that respect. So far, so good. But the possibility of illusions and hallucinations indicates that we can be mistaken, that we may err in our interpretation of those sensory experiences. An interpreted or processed sensory experience or in other words, perception can either be very difficult and then it corresponds to reality or illusory and then we are mistaken about one or more of its properties or hallucinatory and then we're mistaken that there is something external to us whereas in fact, there is no such object at all. The starting point of our perception is not the external world of certain objects or properties as such, but the sensory experience. Our interpretation of that representation might correspond completely or only partially, or even not at all with reality, with the external world. Various philosophers use different names for these pure, uninterpreted sensory experiences. Descartes calls them [inaudible], Locke ideas, [inaudible] impressions, John Stuart Mill phenomena and to 20th century philosopher Russell [inaudible]. Setting aside subtle differences, the basic idea is invariably that the sensations, ideas, impressions, phenomena, [inaudible] are the experiential intermediaries that we, in contrast to the external world, do perceive directly. However, indirect perception constitutes a very serious skeptical threat that we can never stop perceiving indirectly implies that we can never check whether our sensations, ideas, impressions, phenomena, or [inaudible] are veridical. Whether they do, in fact, correspond with reality. We risk getting completely detached from the external worlds. We risk getting locked up behind the veil of perception for all we know, there might not even be an external world at all. Let's now look at some possible solutions to this metaphysical problem of skepticism. Overall, we'll distinguish between three views. Realism, anti-realism, and response dependence, also known as the thought way. According to realism, we can safely assume that the external world exists. There are two varieties. According to Thomas Reid, who argues for direct realism, it's a matter of common sense that there is an external material world and that we do perceive it directly. For most part, the rule is what we experience is what there is. Of course, there are exceptions like illusions and hallucinations but such exceptions prove rather than disprove the rule. John Locke, by contrast, accepts indirect perception and argues for indirect realism. Like Descartes, Locke distinguishes between primary and secondary qualities. Primary qualities, for instance, shape our properties that we experience as they really are. A banana appears curved and it is curved. Secondary qualities, for instance, color or taste by contrast, are dispositional properties. Properties that only manifest themselves under certain conditions in the response of cognitive agents. Banana is only sweet in the sense that because it contains sugars, it has the disposition to produce a sweet taste in a perceiver. According to empiricism, all knowledge originates in sensory experience. An empiricist like Locke solves the metaphysical problem of skepticism by arguing that the existence of the external material world offers the best explanation of the stability of our experiences. Hard-nosed empiricists like Berkeley and Mill by contrast, adhere to anti-realism. They insist that we can only be sure about our experiences themselves and refuse to perform speculative abductive thought experiments aiming to explain them. Berkeley, for instance, argues that if we only go on our experiences, it's impossible to distinguish between primary and secondary qualities. Rather, secondary qualities generalize [inaudible], properties only exists as far as they are experienced. Interestingly enough, as a bishop, Berkeley is so much opposed to materialism that he can't resist explaining the stability of our experiences by performing a completely different abductive thought experiment. Since [inaudible], we ourselves and familiar objects that surround us are not material objects, but rather ideas or perceptions in the mind of God's. However, the hardest-nosed empiricist is John Stuart Mill. According to his phenomenalism, all that we are ever aware of our actual or possible phenomena, immediate experiences, never noumena or substances, never physical objects. In early modern philosophy, we find Kant's transcendental idealism midway between realism and anti-realism. According to the abductive thought experiment that Kant performs, changes in our experiences can only be explained in terms of the existence of an external world. However [inaudible] is not knowable. We can only have knowledge about our experiences. In contemporary philosophy, response dependence constitutes the thought way in between realism and anti-realism. Like Berkeley, it generalizes the idea of the secondary qualities. Unlike Berkeley, however, it refuses to take sides as to what the correct order of explanation is. It's impossible to say whether the banana tastes sweet to normal perceivers in normal circumstances because it is sweet or whether it is rather sweet because it tastes sweet to normal perceivers in normal circumstances.