According to the Mishna, anyone can be made a messenger to deliver a ge% except for a heresh, a shoteh or a katan, as well as a suma and a goy. The heresh, shoteh and katan (the deaf-mute, the imbecile and the child) are normal categories of people who are not considered competent to carry out halakhic requirements. The goy (non-Jew) has no connection with Jewish divorce. The category that the Gemara finds needs an explanation is the suma – the blind person. Why is a blind person ineligible to deliver a geṭ?
Rav Sheshet explains that the problem stems from the fact that a suma cannot recognize the person who gave him the get nor the person who accepted it from him. Rav Yosef – who was, himself, blind, objects that a suma can recognize people based on listening to their voices – tevi’ut ena d’kala – which is what allows a suma to sleep with his wife, and, for that matter, for every person to sleep with their spouse when it is dark at night. Rav Yosef suggests that this Mishna is dealing with a specific case of a get brought to Israel from the Diaspora, where the messenger must be able to testify that he saw the document being signed – something that a blind person cannot do.
The expression used by Rav Yosef to explain how a blind person recognizes people is, “Tevi’ut ena d’kala“. Tevi’ut ayin is the way a person recognizes an object that does not have a clear indication on it of who it belongs to. This is done by means of broader recognition techniques. Looking at a broad collection of indicators, a person can recognize something even if he cannot explain what specific thing proves that it is his – something similar to the concept of Gestalt in psychology.
Ordinarily, tevi’ut ayin refers to recognition of the object by use of visual cues. In our case the term is borrowed and applied to a parallel situation, one where the blind person can recognize people by means of their voices.