Many believe that content externalism is inconsistent with commonsense views about our knowledge of the contents of our own thoughts.' Content externalism is the view that the propositional contents of an individual's thoughts do not supervene on the intrinsic properties of that individual. Relations between you and your social and physical environment partly determine the contents of your thoughts.2 But if what determines the content of your thoughts lies partly outside your mind, it might seem that you have to investigate your social and physical environment before you can know the content of your thoughts. If such investigation were necessary, our knowledge of our own mind would be much less direct and much less warranted than we ordinarily believe. How do we connect content externalism with failures of selfknowledge? I believe that water is wet. This first-order belief about the world is subject to the limitations of ordinary empirical knowledge. But I also believe that I believe that water is wet. Apart from cases of self-deception or conceptual confusion, it is difficult to see how I could be wrong about this second-order belief about my own mind. Even if I am wrong about the world, I know what I am thinking. But if I had grown up on Twin Earth, I would now be confidently asserting that I know that I believe that twin-water is wet. I cannot tell through introspection whether I grew up on Earth or Twin Earth, and there is no qualitative difference between believing that water is wet and believing that twin-water is wet. So how do I know which one I believe? We typically do not investigate our environment, find that water