JDP: If S is justified in believing that p, and S is justified in believing that p→q, then, S is justified in believing that q. (from ppt)

In my point of view, in our daily life, rejecting the Justified Deduction Principle will lead to an “absurd” conclusion, for example, can we deny there is someone in my class has an iPad while I definitely know and justified that Yu Jingdong owns one? Then, according an argument, it will finally lead us to skepticism: we don’t know anything.

Here is the Argument for the Conclusion

1. S is justified in believing p, p→q in ordinary cases; (The Standard View)
2. It is always possible to construct a variation of ordinary cases, in which S is justified in believing p, and p entails q, and S deduces q from p and believes q as a result of this deduction, so S is justified in believing q; (Justified Deduction Principle)
3. If in two possible circumstances there is no difference in the evidence that a person has concerning some proposition R, then either that person is justified in believing R in both cases, or that person is not justified in believing P in both cases; (Same Evidence Principle)
4. If (JDP) is false, then according to 2), it is impossible to deduct and q from p and p→q, so we can say, q is not justified in the variation cases.
5. According to 1) and 2), if S in circumstance A is justified in believing p, p→q, in circumstance B, S is justified in believing q and S have same evidence in these two circumstances.
6. According to 4) and 3) q is not justified in the variation cases, so S is not justified in believing p and p→q.
7. So, no beliefs are justified.