How can I know about any consciousness but my own?
I assume that you are conscious in our sense of the word, and you assume that I am. But what evidence do we actually have to back up, to justify, these assumptions? Are you in fact making an unjustified assumption about me, just because it makes life simpler? Or do you really have good reason to believe that I am conscious, that I have an inner subjective life?
What evidence do you have that I am conscious? What evidence do I have that you are conscious?
My model of my own behavior, the way that I explain my own actions to myself and to others, includes my inner subjective experience and decisions as vital links in the chain of explanation. The only way I can account for what my body does is in terms of my being subjectively aware of certain things, and of my making subjective internal decisions. When I look for an explanation of what your body does, I naturally look for an explanation that includes your being subjectively aware, and making subjective decisions, as well.
I know that I am conscious, and I feel strongly that that fact helps explain the way I behave. You behave, broadly speaking, similarly to how I behave. Naturally, then, I tend to assume that you are also conscious, and that that fact helps explain the way that you behave. But is that assumption justified?
When I conclude that you (and billions of others) are probably conscious, because you behave about like me and I know that I am conscious, I am generalizing from a single data point. Is that generalization justified? Many such generalizations are not.
Are my conscious decisions really part of the best explanation of what my body does? If not, and in particular if the actions of my body can be explained entirely in objective terms, my reasons for concluding that you are conscious seem shakier still. I am left with "the particular set of atoms that makes up my brain is associated with a consciousness; the atoms that make up your brain seems sort of similar to mine in various ways, therefore there is probably a consciousness associated with them as well." This seems far from satisfactory!
What evidence do I have that the ways that our brains are similar are the right ways? Since we know so little about the relationship between consciousness and the physical world, how do I know that consciousness does not depend crucially on one of the many things that make your brain different from mine?
What if I had a brain-reading machine, that would somehow let me experience what you are experiencing, or that seemed to be giving me access to your consciousness? What sort of evidence would be sufficient to justify my believing that that was actually what was happening? (Rather than, say, the machine simply causing me to experience roughly what I would have experienced if the same thing had happened to me, regardless of whether or not you actually had subjective experience of it at the time.)