Don't say the Shema . . . until you can distinguish (Berakhot 9b)

From what time do they recite the Shema in the morning?  asks the Mishnah.  It provides an answer based on the ability to distinguish colors.  From the hour that one can distinguish between blue and white.   R. Eliezer says, “Between blue and green.”  The Gemara goes on to quote two other traditions based on the ability to distinguish between similar animals: “a wolf and a dog.” (R. Meir)  or “an ass and a wild ass” (R. Aqiba).  But these are rejected in favor of an opinion from the Tosefta (ascribed simply to “others”): “Once one can see his fellow four cubits away and recognize who it is.”

All of these options are subjective and imprecise, giving a feel for daybreak rather than a definition of it.   If the last one is preferred, it's not because it's more effective in a practical sense ("Harry, go step four cubits back and I'll see if I can recognize you").   I think it's because the ability to recognize a person is morally fundamental in a way that distinguishing species or colors is not. 

I find it interesting and appealing that we are concerned with being able to make distinctions at the same time as we're affirming Unity.  Going into the Shema I am reminded that yes, Harry and I are different, but our life flows from the same Source and is grounded in a single Substance.  Coming out of the Shema I've affirmed that it's all One, but look, there's that One reflected in Harry as it is in no-one else.