Comparing Putnam's Skepticism And Content Externalism

In section 2 of Skepticism and Content Externalism Hilary Putnam argues that she can establish she is not a BIV on the basis of semantic considerations alone. Putnam uses a martian on mars in comparison to a brain in a vat to prove her argument. Putnam states that if the Martian forms a mental image of a tree in reference to an illustration of a tree it is actually not a representation of a tree. This goes for the BIV as well; The martian and the BIV do not have casual interaction with trees that allows them to recreate an image that refers to trees. When comparing the skeptical argument and Putnam’s Argument there is slight confusion/contradiction. According to premise 1 if you know that p ( here is a tree) than you know that you are not