Aside from fears that cameras might actually steal souls, there is an inherent tension between privacy and photography.  That tension plays out daily in the world's cities.  Obtaining consent would seem to respect subjects' privacy rights. But not many great urban photographers have done so.  Diane Arbus was an exception; she got her subjects' permission and then openly photographed them.  Her approach made them collaborators, rather than subjects.   Other fine photographers were not nearly so open, and at least one, Walker Evans, took non-consensual photos on New York subways with a concealed camera.  His subway photos are not collaborations; they are, if anything is, true "captures." And Garry Winogrand was recording, prolifically, whatever was happening in front of him.

Stopping to request consent on a Paris or New York sidewalk would stop events in mid-flow and harm spontaneity, even if did not irretrievably transmute something interesting into something vapid. 

So consent is problematic, but not necessarily impossible.  I took this photo with the subject's consent.

Bistro Patron, Paris, 2010

And I asked the three carabinieri for permission.  It seemed like a good idea.  

Carabinieri, Florence, 2011

But consent would have been impossible for these photos.

Shoppers on a Mission Outside B.H.V., Paris, 2010

Cyclist, Paris, 2010

And perhaps she would have stopped and given consent (after all, I was using a 24mm lens and was pretty close), but the dynamism and intrigue would have evaporated.

Red, Rome, 2012