(The view, which involves the idea that we have no good reason to believe that our perceptions of the world are veridical, is called external world skepticism. ) External World Skepticism is the thesis that we cannot know what the world outside of our minds is like. Here are two hypotheses: Hypothesis1: the external world causes us to have veridical experience. For example, a tree causes me to have an experience of a tree when I look at it. These experiences are veridical. (This hypothesis assumes that common sense realism is true).
Hypothesis2: I am a brain in a vat, and an evil scientist causes be to have non-veridical experiences. For example, he can cause me to have an experience of a tree that is qualitatively identical to the experience of a tree explained in thesis 1. These experiences are non-veridical. (This hypothesis assumes that common sense realism is false). Here is a precise statement of argument for external world skepticism: 1) If we have two hypotheses that postulate qualitatively identical experiences, then we can’t know on the basis of experience which of the two hypotheses is correct.
2) Hypotheses 1 and 2 postulate qualitatively identical experiences, so we can’t know on the basis of experience which of hypotheses 1 and 2 is correct. –from(1) 3) We can’t know independently of experience which of hypotheses 1 and 2 is correct 4) Therefore, we can’t know which hypothesis is correct, period. from(2)and (3) 5) But knowledge of the external world(the denial of external world skepticism)entails that we know that hypothesis 2 is incorrect, and common sense realism implies that we know that hypothesis 1 is correct.
6) Since we can’t know this, we can’t have knowledge of the external world. This is External World Skepticism. Premise(1) is very plausible. How can experience alone select between hypotheses that predict the very same experience? (2) follows from (1) together with our hypotheses. Premise(3) seems very plausible. On what ground could we distinguish between these hypotheses if experience can’t? (4) follows from (2) and (3). (5) is a logical truth; it follows from the definition of external world skepticism(or from the definition of common sense realism).
(6) follows from (4) and (5). Descartes’ skepticism- specifically, the problem of external world skepticism—is not a full-blown skepticism concerning everything. Descartes tried to defeat external world skepticism. Descartes is going to attack premise (3) of the argument for external world skepticism. He proposes that we have knowledge of a very important fact( knowledge that we come to have independently of experience) that allows us to see that our sensory perceptions must be veridical.
That is, knowledge of this fact entails that (very probably) there is not an evil demon that systematically deceives us. There are two steps of his solution. Step1: Descartes believes that there are sound versions of both the cosmological and ontological arguments. The following is a simplified version of Descartes’ cosmological argument. He believes that we know the following principle a priori: the cause of an idea must be as perfect as the idea that it represents. Consider our idea of God as the perfect being.
According to this principle, it follows that the idea of a perfect being could only be caused by a perfect being. Hence there exists a perfect being. Step2: Now that we know that a perfect being exists, it becomes rather easy to avoid external world skepticism. A perfect being would not want all of its creatures to be systematically deceived about everything (other things being equal, true beliefs are better than false beliefs), so the perfect being would design things such that our perceptions are veridical experiences, and that it’s not the case that an evil demon/genius deceives us.
Hume provides an objection to Descartes’ solution. Hume’s basic idea is this: different kinds of statement require different types of justification. There are two different kinds of statements- those expressing relations of ideas and those expressing matters of fact- they must be justified in different ways. Thus, relations of ideas are a priori; matters of fact are a posteriori. According to Hume’s claim, all and only relations of ideas are justified a priori; all and only matters of fact are justified a posteriori.
This position is called empiricism. It says that there are no synthetic a priori statements. (Rationalism holds that there are some synthetic a priori statements. ) Hume’s claim entails that there are no synthetic statements that are justified a priori. This alone should be sufficient to cast doubt on Descartes’ solution to the problem, for his strategy is to deny premise (3), which commits him to a synthetic statement that is justified a priori.