>I think one needs to have some sort of inner vision as well.
I don't know what the previous sentence was, so I'm not sure what Sacks means by "as well." "Inner vision" could mean something like "insight." I assume he means that you need to both observe things (vision), and understand things ("inner vision).
>When I was thinking and writing [...] who would often come along with me.
Here, Sacks implies that he was traveling and studying deaf people, in order to write about them. He had a friend who was not deaf, but who had deaf parents. This friend often went with him on his trips.
>When I went to [...] who himself was born totally colorblind.
Sacks traveled in order to study colorblind people. One of his companions was a colorblind person, who was a physiologist.
>In this way, there can be no condescension or looking at a distance.
Sacks is saying that he could not be condescending toward deaf/colorblind people, because his own friends belonged to the deaf and colorblind communities. So he could not view deaf people or colorblind people as "different" or deficient. Even though he was studying these groups, he was not studying them "from a distance," as if they were some kind of strange, foreign subject. He wrote about them as part of his own culture and world.
>But now my own cancer is a sort of mediator [...] I too am a patient.
A "mediator" is something that makes a connection between two things, by coming between them. Now that Sacks has cancer, his patients feel more connected to him. They feel like they are similar to him, because he is a cancer patient.
>Although in some sense, we’re all patients.
Sacks seems to be suggesting that everyone has physical or mental differences that could be studied and perhaps treated. There is no such thing as being "normal." We are not all literally "patients," but everyone has differences or flaws that could, potentially, make them a "patient."